if your in the UK and its late here in the states.. man you ARE a nightowl. Frankly don't worry about the screws man. Go find some machine screws that are 1 1/4 longer than your baseplate including head measurement. I don't know the specs so humor me. The head should be able to be fully screwed in and not rise above the plate it is being screwed into. So you will have 2 depths of cutting... the first that is cut to accomodate head size and the 2nd through the center to accomodate the shaft. The only tapping should be on the target plate, the head recieving plate need not have any threads. I can't tell you any measurements as I don't have any provided via.. you. Any machine shop should be able to measure standard machine thread, and measure width of screw radius. Machine shops need only see the screw to easliy make a tap for it. Make sure the screw goes in.. 3 mm as a guestimate + the 1mm-2mm gap that the o-ring will rise above block. I cant give you, that as I dont know how thick an o-ring you are using.
See? I dont think anyone can help with that stuff cept you provide the exact specs. And that is a time waster. Just bring your 1 1/4 machine screws to the shop and say "put this in".
Bring in 3-4 o-rings to the shop. Make them of varying diameters, as he could cut the ring differently.
Man.. its late.
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