The red dot on the grip is the button you were asking about?
Huh - the only thing I can think of is another safety - I'd have to look at my mag but it might be in the right place to block the back of the rod that connects the sear & the trigger.
Why not just pull the grips off & check? there's nothing you can mess up doing that, the grip frame is empty.
"Now is no time to be making new enemies" - Voltaire (When asked on his deathbed by a priest to renounce the devil and turn to God.)