hi John
The cheese head screws that hold the feedscrew nut are 4 BA. Probably 3/8" long. Those are what you mean.
When my brain saw "top of the slide" it stuck on the feedscrew bracket as that's on the bit that slides up and down. Any other words ignored . Funny things, brains. 
In Imperial sizes Myford went for 2 and 4 BA for screws and bolts less than ¼", then either BSW or BSF, but to be contrary you might find a 3/16" BSW somewhere.
Caveat: on old Myford kit there is always the possibility that someone has changed things.
Edited By Robbo on 14/11/2018 10:23:50