that is what happen to mine, 1957 TO-35,
my old bush hog pto shaft worked loose and rode up the tractor pto shaft and ruined the threads on the housing.
lucky for me it did not come off
but I have never been able to get the pto safety cap to screw back on, its been that way for years and has not been a problem.
on the to-35 you do not have to keep the pto turning when you are using the lift.
if I had to keep the pto turning all the time, it might be more of a safety problem.