Rear sight holes . What screw ?

good question, I would like to know as well... I put a Williams peep sight on my HW30. I once saw an image of one with some sort of filler cap in the empty screw holes. Hopefully you'll get your answer.

I think you will find the holes are for the old diopter rear sight and they function as a positioning point

https://www.google.com/search?q=weihrauch+diopter+rear+sight&safe=off&rlz=1T4GUEA_enUS644US645&tbm=isch&source=iu&ictx=1&fir=YZO0S2RmI8uTdM%253A%252CDQdPvu8eq54HOM%252C_&vet=1&usg=AI4_-kTka_yKqzklqfDKHvDcm1s_k48TUw&sa=X&ved=2ahUKEwjcidjBiZbnAhVGqJ4KHYQyCTUQ9QEwAXoECAoQBQ#imgrc=YZO0S2RmI8uTdM:&spf=1579657165952&vet=1

Metric. Size M5x0.5

There used to be some nicely blued blanking plates made to cover that area. They came with screws to fit the existing holes. I think they were english made. Beeman, among others, used to sell em.

I have found no source of any type of screw that can be used, you can google filling in rear sight holes and you will find many ideas. I have one, that instead of finding the right screw, you go to the next 1 or 2 sizes smaller in a nylon screw, you can get them in black, pan head, button head, phillips, slotted, and just cut the thread to a length that will not bottom out, and just stuff it in. As long as there is resistance, it will not fall out. I know that the OD of the M5 is just over .187, and that an 8/32 nylon bolt should work to fill the hole. You could always use some silicone if it was not a snug fit. I will bring home some 8/32 button head cap screws, the tap drill for the 5mmx.08 screw is .165 or 4.2mm. The outer diameter of the #8-32 button head screw is .160. I know by MFG they can vary, but a #8 screw should go in the holes without doing any damage to the threads, and just a little silicone, or other non permanent glue should hold them in place.

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I checked my old R9 sight adjustment screw and it does seem to be M5 X 0.5. You could buy these cap screws, cut off the end, cut a screwdriver slot with a hacksaw and cold blue. Or you could see if you can buy them from Weihrauch. To modify screws I just run them into an undersized hole in a piece of hardwood, then cut, file or whatever. To cut the slot I would make a hardwood jig with the hole for the screw and a slot to guide the hacksaw blade.

https://www.amazon.com/0-5mm-Pitch-Thread-Socket-ABBOTT/dp/B07BFMY1DYT