Master cylinder screws.

[joebloggs]

I can't confirm the size, but would agree with your guess.

I took my originals into the local bolt suppliers and they were able to match them easily, so they aren't anything odd.  I switched to ones that need an allen key rather than screw driver, less likely to get chewed up.

+1 on the allen key, stainless with a little copperslip will make sure they come out next time