My experience in case it helps other who happen on the tread…

I’ve struggled with MVA screw choices and had Cheryle send the wrong ones no less than four times.  Length of phone call made no difference.   Or maybe it did.   Perhaps I just talked her into confusion.

8-40 for Cody Ballards .161
9-40 for Original Ballards .169

9-40s have such a tight tolerance that you sometimes have to hold your mouth right to get them to start.   And wonder if you have the right size sometimes before you get your top lip curled just right.
8-40s feel sloppy but they will take torque sufficient to be solid.   At least they have in some of my Ballards for a number of years.

The .161 & 169 (above) are what I just measured on my MVA screws.   I have not pulled original Marlin Ballard Tang sight screws and measured those.   That would be the authoritative number in my books.

Why?

Because the tolerance of theirs block halves screws learned me there is nothing like those guys in a Cody (i.e, modern Ballard).

I’ve had block screws back out on Cody Ballards in the middle of a match and lock up my block.   Never in original Ballards.    I can finger tighten a Cody screw where I need a screwdriver right away for an original Ballard.  I might could do it with my fingers but there is resistance immediately from the close tolerance.

Anyway, that is my experience.