Shifter and TC Cover question

I’m replacing the gaskets on all the covers of 1970 580b CK. The cover on the TC has 3 bolts with 9/16 heads but the 4th one on the right side closest to the shifter is sunken into the cover. 3/8 was too small, 7/16 too big and 10mm too small, 11mm too big. Does anyone know what size this should be? Second in the manual it warns about the detent balls flying when removing the shift rails. If I take the gear shift cover off to replace the gasket will these come out or are the shift rails still going to be attached? Thanks all