From what I've heard to use those mounts on the older Optoma projectors you have to reverse the bit on the end of one of the legs (the shorter ones maybe?) to make it fit.
EDIT: I think the Optomas can only mount to 3 of the legs as well on those - the 4th hole is like on cameras, etc. for mounts that have that as the central point.
when my JVC was on a mount, you had to turn it upside down and unscrew the feet and use those holes
Are you able to maybe make a simple 'image' showing what arm to take off etc and i will do it and see what it looks like?
Looks like the projector has 3 mounting holes.
Just remove one of the legs from the mount and position the other 3 to fit.
I had to do this with my mount on my benq, also had to use a few washers to get clearance etc but most mounts are universal these days and with playing around it sound fit.