Rossi m92 pre D/T holes under rear sight

[Blake6422]

Hello,
I have a Rossi R92 in .38/.357 16",
Under the rear iron sights are 4 pre drilled tapped holes, I was wondering if you would have any clue what size thread/bolts are needed for this?
I am planning to mount a generic rail to it, but I am stumped with what size bolts I need. (The bolts that came with the generic rail are too big)
A m3 is too small, m4 too big,
A 1/8 screws into it but is very loose (it screws in enough that you cannot just pull the screw straight out)
So I'm thinking it may be either a 9/64 bolt or a m3.5?
Have you ever come across this situation before for this rifle?

I've seen the actual braztech rossi bases specifically made for this rifle, but EVERYWHERE I've looked they are either out of stock or the supplier has stopped making them, if anyone has this base, could they please measure these bolts, or if there's some sort of markings on them promoting what size they are would be a great help.
Thanks, Blake