EOPS, attorney Keith Langer and the late attorney Darius Arbabi all disagree with your interpretation. There is a reason that they came to their conclusion.
There's something called the rule of lenity which effectively states when there is a vagueness, the tie goes to the citizen. Lenity is the construction favored by the SJC when dealing with ambiguity and vagueness. "readily adapted to" is what one would call ambiguous and vague...
But more importantly, it was the desire of the anti gunners to effectively ban semi auto's over time. But they didn't want to ban them by name because by doing so, they would have a more difficult fight on their hands then simply arguing capacity >10 was evil. Lets face it, the >10 is evil resonated back then and is resonating today with the supposed pro gun crowd (bob levy, dick cheney, etc) whereas a semi auto ban would not. So in splitting the baby, they introduced a lot of ambiguity/vagueness and as a result this opened up the door for the interpretation we have today.
Remember, legislation begets regulation. When 1998 happened, a lot of good people busted their butts to have EOPS interpret the statutes when creating their regulations as broadly as possible. It is my opinion that the legislature intended a B to not cover semi autos and the ambiguity in this aspect (and the carry aspect) is why I believe glidden counsel's against writing class B permits. So these people did a good job in making sure class B holders could get at least SOME SAs like 1911s and the such. They also made it so that some penalty enhancements don't apply in the cases of single stack SA firearms.
The rule that single stacks are OK for class Bs and that the capacity of the firearm is determined by the existence of OEM >10 mags or not has created some weird results but this is how EOPS interpreted the statute. It's effectively going to give class B's a lifetime much longer than it ever deserved but it is in keeping with the ambiguity in the statute. If the legislature had intended for hi-cap to equal semi auto they should have said that, no?
Anyhow, the weirdness I mentioned is this. DWM made a Luger with a 32 rd drum magazine and a stock. Yet Lugers are routinely sold as <10 cap single stacks (and the ATF exempts lugers from their AOW regs BTW...).
There are aftermarket 1911 mags that hold 11 or more rounds. Take the case of Glock 27s, 29s and 33s. Are they high cap? They can't hold more than 10 rounds with the OEM mags?
Reconcile that. In reality you can't because it's a direct result of the ambiguity and vagueness in the statute.
I believe that if EOPS had interpreted the statute as intended, they would have classed out SAs but by tying the "or readily adapted to" clause to the existence of OEM mags, they sent the tie in our favor.
Long way of saying I believe that in the vacuum of the legislation without the additional EOPS regs and interpretations, GSG is right.