M8.. hmmm. Out of my comfort zone, now, as far as who sells a quality gage with those threads. I think your right, either M8 or M10.
Not that they are bad, but I like to put on gages that are as close to the max fill and regulator set as far as readings go. Like on my impact I changed both to 0-250
It is 8mm by 1. Pretty stupid sizing if you ask me. I tried a adapter but I don't think it is near correct type as it leaked. Almost makes me want to drill it out and put a standard pipe thread in there. And I bought a 10mm x 1.0 thinking it would fit. That was before I actually removed the guage to measure. Nope did not fit, should have measured before I bought.
Just for reference this is on a Lcs sk19 bought in August of last year though.