OK, 1 makes sense, thanks.

The text says that there are two 2BA screws, and the opposing holes in the feed ring are, from the looks of it, also 2BA, so I assume that the two larger screws go in these – and that makes sense if you want to have two "clicks" of advance per rev, while withdrawing one of them takes you down to 1 "click" per rev. Which all leaves the 0BA screw looking rather redundant?

If you're referring to the photo on p13, the way I see it is that one of the 2BAs is in place in the ring – bottom left of the ring. The 0BA screw is too short to go through the feed ring and project into the groove, and having the thread rub on the cam ring would not be a good idea surely.

It seems to me that the first sentence of the first paragraph that starts in the r.h. column of p13 makes it clear that there should be 2 off 2BA grub screws.

And, of course, I have been thinking awrong. I had been seeing the drawing of the 0BA screw as having the smaller diameter threaded, and being mislead into assuming that 0BA is smaller than 2BA. One day I'll get my head around the idea that with BA, SWG and number drills, a bigger number is a smaller thing. ​ Having things to different scales in the same drawing doesn't help. But yes, I'll go with the 2 off 2BA – though actually they will be M5 as I'm much more at home with metric (see above).

Thanks for the help.