If you aren’t seeing the other mounting option, odds are someone on the team you got it from has cut it off already. (The motors are supposed to have a stud on the face plate, to which a belt tensioner attaches. You could remove the tensioner and retain the stud.) Clamping is a pretty good option, though unfortunately, I don’t think these motors had an alignment boss around the shaft.
As I recall, the front face was somewhat teardrop-shaped, giving two flat sides that could be used to keep it from rotating. I know 330 clamped theirs by bending allthread into a U shape of the proper diameter and setting them on a light-duty platform, then using the custom Ubolts through it, back in 2006, and as far as I know, it’s still there. (Admittedly, this was a shooter motor–but we never did have any issues with the shooter not working once we got it working.)
I used these motors on a project a few years ago. The way that I mounted them was there are 2 bolts that hold the motor together one when stock will stick out ~1.5" you can replace the 2nd one with a longer one so that you have 2 bolts to mount from. The thread is an M6 but I dont recall what the pitch is but I can look it up if needed. I also cut the profile of the face of the motor into the plate that it mounted to. Below is a link to some pictures showing what the profile was. I can send you the CAD model of the plate if you would like.