Did some searching here on pinside and have not found any wisdom on the subject.
Never seen or heard of this until i went to the pinball resource page to look up parts and saw this note http://www.pbresource.com/rebuildkit.html#gtbma988 with a link to this tsb form Permier/Gottlieb in 1987 http://www.pbresource.com/rebuildk/gtb-ma988MA989.jpg
The ma-988 and ma-989 conversion kit was a field installed fix to solve complaints to the premature wearing out of end of stroke switches on system 80b games.
So yes 1st thing was to go check my current games again to see if this has been done to see they have not been done. I'm here to see what others have done and hopefully inform other gtb owners to pay closer attention when rebuilding there flippers.