I notice there seems to be a change in the mounts, that are drlled through frame and welded in.
The earlier bikes had a flange head bushing welded in , head is aroud 15mm for a 8mm hole.
On a 78 i checked it eeemed much smaller like maybe just 12mm straight tubing.
If you have a 76 or 77 frame without mount on coukd you please check and report. Trying to determine when change happened. . THANKS