These top sash guides are going on your 305 Spiral 3/8 in diameter Balances. It appears they were 3/4 in wide across the top as they are a bit worn. 15/16 in from top to screw hole. Let me know if you need additional information.
Casey from SWISCO responded:
December 7, 2018
Hi there, I would check out the dimensions of our 18-034 top sash guide. Let us know if you think this could be the part you're looking for.