Short answer: it really depends on who you ask. If you ask any judge in CA, then no.

Let me preface the long answer with a bit of linguistics. "Baton" literally is "stick" in French. ASPs, Monadnocks, and the like don't have a whole lot in common with them thingies as grow on trees. But they're still called batons because that word in contemporary English can basically refer to, among other things, any short simple bludgeon. Back when that CA law was written, "billy" was the word people used instead of "baton" for any short simple bludgeon. The most common type at the time just happened to be the sort in the pic and in the NY decision. Just as the word "billy" went out of fashion, so did that particular type of smacking stick. To narrowly define "billy" as only the sort of truncheon carried by bobbies in 19th century England is to assign a specificity that didn't exist at the time.

Of course, there have been myriad definitions of "billy" as a striking implement in different places and times. Nowadays, billies are commonly understood to mean the bobby's wooden club. But it seems at least one old definition of "billy" is that of the flexible whacking weapons, which we would now call saps, commonly used by unsavory folk at the time. The online etymology dictionary lists the oldest definition of billy as a crowbar. So really, it just depends on who you ask. http://www.etymonline.com/index.php?term=billy

If we're going to go back into the etymology of the word (of which I was aware but good on you), then there is very strong evidence that it applies specifically to clubs given to law enforcement personnel. From what I've found now and before, "billy club" originated from the clubs used by a "Billy" referring to the law enforcers of the time who worked under King William IV, hence the moniker. Such clubs were not just weapons, but were also specifically used as a badge of office as well. The image of a billy club is still so strongly associated with police that even kids know what a billy looks like and who has it.

A few years ago, the relevant law was CA Penal code 12020. I need to verify, but that seems to have been replaced with the codes I linked earlier. So if we do assume that billy was a generic term at the time of the law-writing (the last few years), it still has to mean a smallish carven-handled wooden club used by police. Considering the fact that they listed numerous specific weapon names, going so far as to include sap and blackjack, it looks like they do acknowledge the specificity of naming which would mean that if a baton were illegal to carry, it should be unambiguously listed as such. If given a baton and a billy, I think that most people would name them differently; but I doubt that most people would know the difference between sap and blackjack.

Check out italicized paragraph under the picture "Constables did not begin wearing uniforms until 1829 or carrying warrant cards until the 1880s"
http://www.neatorama.com/2013/03/15/Decorated-Police-Billy-Clubs/

http://law.justia.com/codes/california/2005/pen/12020-12040.html

Fun side-fact: Silly-billy may have maybe also come from King William IV himself. Or a cousin or something. "Who is silly billy now?"

Not sure if you noticed, but the etymology link you linked says that billy club didn't mean crowbar literally:
"originally burglars' slang for "crowbar;" meaning "policeman's club"

>>18211
Dude, the California Penal code was written in 1872, not the last few years. Each time it's edited (which is whenever a bunch of lawyers are uncomfortable with their job security) the revision date is noted as the date of the law since only the latest version is at all relevant. The language of that section, and thus the definition of "billy," dates from at least 1872.

>>18214
Do you know the difference between a comma and a semicolon?

>>18216
http://www.halloweenexpress.com/billy-club-p-8318.html#.

I'm not attacking you there, just pointing things out. Since grammar is very important when examining law, let's look closely at the semicolon for practice. It is being used in one of two ways. Either it is joining the "slang" and "meaning" parts together like this: "originally burglars' slang for "crowbar;" meaning "policeman's club"" or it is being used as a part of a list as a sort of "greater comma" like this: "Here are three examples of familiar sequences: one, two, and three; a, b, and c; first, second, and third." If it is the second, then it is not related to burglars' slang, and it is readable as: "club, 1848, American English, meaning "policeman's club"". Either way you look at it, it does mean "policeman's club". So to answer your question: yes, I know the difference between a semicolon and a comma, now you do too. Now take a breath and relax. I'm not out to get you. If you have anything to prove your point I would like to see it. I can't really put it any more politely.

Yes, the CA penal code was adopted in 1872, but it was updated in the last 5-10 years. Because it was updated, we can look at the latest revision as applicable "since only the latest version is at all relevant". In the latest version (as I pointed out earlier) we know that the law is very specific in scope because the difference between a sap and a blackjack is not big, but it makes that distinction regardless. It even goes so far as to include both "sandclub" and "sandbag" both of which are obviously quite similar. The sand ones are more similar that "billy" and "collapsible baton", thus one can only infer that they intentionally left out the batons.
Additionally, for it to be banned by the CA penal code, a collapsible baton must be an "instrument or weapon of the kind commonly known as a billy, blackjack, sandbag, sandclub, sap, or slungshot". This means that the club part is not the important part, but the "billy". And as the historical data shows, the law has changed in innumerable ways, but the popular and even the legal definition of "billy" in this context is and has been clearly and strongly defined for centuries. As I said earlier, it is so strongly ingrained into popular knowledge that it is part of halloween costumes even for kids (see earlier img). Ergo, vis a vis, concordantly, quid pro quo, Clarice, the definition does not cover collapsible batons because the law simply does not mention them.

A baseball bat is not a billy, a tire iron is not a billy, a nunchuk is not a billy, a tree branch is not a billy, a chair leg is not a billy. Why, then, do you think that a collapsible baton is? If they meant weapon, then they could just have put "blunt, striking weapon", but they didn't.

With all that said, are there weapons that are totally legit legal to carry? The CA code doesn't have the "bludgeoning instrument" (or whatever) part that the NY code does. If nothing else I would think that a walking stick should be fine. Someone involved in law once told me that just having a bat or similar in your car is proof that you are going to commit a crime. But I don't have proof either way.