I'd like to thank everyone for their comments. But I have somewhat solved the problem. I unscrewed the set screws on the Petri t-mount and an old Nikon t-mount I had, and removed the center piece with the t-mount threads. I saw there was little difference in in the size of the rings, so with a little grinding and filing, I got the thread ring from the Petri mount to go into my Nikon mount. I tightened up the set screws and threaded it back on the lens, re-set the screws to center the lens, and there we go, mounted perfectly on my old Nikon FTN. One leetle problem; the lens won't quite focus to infinity on the camera. It's good to about 25ft. or so, then that's it. I may work on this problem some more, or just use it for closer subjects.
Life's a learning experience, no?