The thing about the MTM knobs is the shaft which I believe is not consistent with any other Simmons knob. I don't have an MTX9 or SDS1000 but I think they are quite like the SDE knobs but with a longer shaft, whereas the SPM knobs don't have a shaft at all.
Mocked-up an MTM style knob from a picture of an MTX9/SDS-1000 one. Would definitely need to see one in person to get accurate dimensions though. Photo added to 3D Printed Simmons Parts Album.