Mark -
These later belt-drives are nearly identical to your 1960 WP, with one major difference, one which I consider good and bad together.
The centerpost was shortened in early 1978 by five inches. The agitator shaft remained the same, but the spin tube of the basket drive and the centerpost on the baseplate became very shallow. The spin tube went from 18.5 inches long to 13.5.
This was a savings in materials, especially when multiplied times the millions of machines made from 1978 to 1987, AND it essentially all but eliminated bearing wear, which is so prevalent in the 1977 and older units.
That's the good part, the bad is that though these centerposts are more heavily sealed than the previous generation, when the seals fail, water pressure from full or nearly filled tubs press the air trapped under the agitator into those seals. Worn seals allow the air to escape, and replace itself under the agitator with water. Once water gets into the bearings, it starts to seep down past the basket drive and directly into the gearcase.
This can happen on the older centerpost models, but it takes much longer as the taller steel centerpost prevents water leakage until the seals are really bad on those models.
The damage caused by this type of leakage can be ugly. Black oily water, from the centerpost oil mixed with detergent and dissolving rubber seals, can spray out of the top of these short centerposts and make a black drippy line on the inside of agitators, which can eventually get on clothes. This is easiest seen on a machine with a white agitator.
On the other side of that, when the gearcase gets water in it, it fouls the oil like water in a car engine, and it becomes this brown soupy mess, that looks similar to chocolate milk. Eventually that stuff spews out and gets carried away by the rapidly moving belt and will coat the cabinet and everything else in a nice brown oily haze.
I don't know if this was happening to your machine or not, as the one cool testament of these machines is that they seem to continue to operate fairly well while being slowly poisoned to death. You may indeed have had just a loose belt, but most all of these machines eventually need a complete tune up (cleaning, lubircation, and a re-seal) which is not a minor job. Weak floors seem to hasten seal wear, and I've had to do total rebuilds on some of these as young as 8 years old. On the other hand, I know some have exceeded 25-30 years in continual service.
On the bright side, I have quite a number of these machines, several in fact of the very model in this thread, and if you'd like one to work on, its yours, just come get it.
Gordon