I went into a good hardware store that had small bins of screws of diverse types, one of which was metric slot-heads in an assortment of sizes. I simply grabbed one each of the smaller ones that looked plenty long for trimming to length. If I had to guess it was 1.6mm, but it could have been as big as 2mm. If I had been smarter I would have taken the original screw with me and could tell you which it was. As it is I've got this bag of half a dozen assorted metric screws left over, and who knows which is which.
The one that worked is either galvanized or cadmium plated, so it actually looks bad on the gun. It's so small that there isn't much hope of stripping and bluing, so I'm now in the hunt (probably Brownell's) for a blued one. The trip to the hardware store was a cheap (9 cents each) test of solutions.
Wish I could be more specific for you. I said "good" hardware store because this place isn't Home Depot or one of its clones, rather it is more industry-oriented and serves commmercial fishing boats.
Hope this helps.
