A buddy bought his a few years ago here in Cali. Besides charging more for it, it came with the 2 blocked magazines and a "muzzle brake" - more like a chopped down flashhider, silver soldered to the barrel.
It should be just like the M1A - legal with a muzzle break, not legal with a flah hider. The law in Cali defines an assault rifle (quoting loosely) as a semiautomatic centerfire rifle with a detachable magazines AND any one or more of: pistol grip below the receiver, flash hider, folding or thumbhole stocks (this is the 2nd "feature" list; there is also the inital "named" that preceeded the "feature" list, and which is still applies as well).
I have never seen a bullet button for a BAR, but the button elimates the intial detachable magazine, which then allows everything else. For the BAR, the only thing that it has other than the detachabel mag. is the flashider, get rid of it and her is no "and" item. It has a detachable magazine AND none of the other features. Not an issue.
A 1919a4 is legal in Cali, but a 1919a6 is illegal / considered an assault rifle. Cali law defines a "magazine" as "any ammuniton feeding device" which would include a belt. Add a flash hider and you know have a detachable magazine (belt) and a flash hider = assault rifle, which, if unregistered, is a felony.
Getting back to point, I don't see why they would have to fill the mag well on BAR, unless it is just a case of CYA, which they have every right to do.