Ok,
I’ve been in touch with company that supplies RHP pillow blocks,
and they refer to them as set screws,
I need a 5/16th UNF set screw,
I’ll try Alan’s in Netherton a short walk away before going down route of ordering off internet,
thanks for help 👍
the pillow bearer/block is between the coupling and stern tube and holds the prop shaft in line,
the set screws hold the inner race of the block to the prop shaft and then spin with the prop,
with my set up (using a fenaflex coupling) the screws also help prevent the shaft from sliding back and forth,
if that makes sense