Check Calguns.org forum for details on what is legal in CA as it sounds similar. Folks there have come up with the "bullet button" mag release which is a mag release that is recessed and can only be released with a tool, such as a bullet tip so it is technically not a detachable mag. There is a version which has a lock you so can lock it for only "tool" operation within CA, or unlock when you are out of state.
Also they have the MonsterGrip which is not thumbhole or pistol grip. It attaches where the pistol grip would but blends into the buttstock. You can't operate the safety with your shooting hand thumb any more, but it then allows detachable mags.
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