There's one thing I came to think of... The black screws aren't really black. As boss freak pointed out earlier they're merely painted or made black somewhow. Is it possible that the screw in your HM-2 has simply been stripped of the outside layer and that it actually used to be black?
No, there are right angles on mine (just like on a Floyd as I wrote earlier), the black ones are not like that (if you look close). they are not 90 degrees from top to side. But I will let my son take a picture of it as soon as I see him (if itīs possible take a picture of such a small thing). I can try pdf-scan it here (donīt know if it gonna be good enough). BUT donīt let us do too much of this, IF there was a transitional screw somebody must have noticed earlier. But as it was exactly the same lenght.....
Officially the HM-2 was out 1983. I expect that the last silver screws was used 1980. I've never seen any of the 4 pedals released 1981 with silver screws. I also expect the black screw then to arrive 1980.
Your HM-2 is #654500. Mine (with a normal black screw) is #441300. I bought it brand new late 1984 or early 1985. Which one was made first is anybody's guess.
Got the scan of the screw. I would guess that it is in fact a replacement from a locking tremolo system, another brand of pedal or from a hardware store. I've never seen one like that on a Boss pedal.
I've had a look at the scan now and I don't recognice that screw from anything. I have no idea where it may have come from but it doesn't look original.