1994 Mazda B4000 Donnelly rear view mirror

Like Anton in '07, I can't figure out how to separate the 'button' (or base plate) from the mirror's mounting bracket--and the solution(s) provided don't seem to apply to Model2410. There is what appears to be a 'key'-bar beneath the wedge-shaped plate itself, which if somehow could be moved to the right, would line up with the channel (referred to as a "gap" in the afore-mentioned q&a series), enabling the base plate's removal thereby. No amount of pulling or twisting, nor insertion of any tool into said 'slot', has benefited. PLEASE help, if you can and would. Sincerest thanks, in advance!

PS: Scrolling down, I see that a donation may possibly encourage an earlier response, as is understandable. Being disabled seniors and therefore among those who "can't afford professional advice", I (&the Mrs.) Nevertheless can and will offer $10, with hopes that 'you' wouldn't be offended by the paltry sum! Again, thanks VERY much!