I'm going ahead with a left-side 'cartridge counter' grip & magazine. I'm planning on using an extended follower pin/button for the indicator.
I didn't have a spare Mec-Gar mag, so I ordered one from
Greg Cote. I pressed out the base pins with an arbor press. There seems to be a correct way to do it, as shown in pic. After moving ~3/32", they popped right out.
Magazine base and follower are plastic, so I'll have to make metal replacements.
Locating the follower pin hole [cartridge indicator] in relation to the body, both 7-round [full] and one round [end of proposed slot]. 'Full' pin location is with top cartridge under tension from breechblock.