Interesting Regulation of Yesteryear

[Diver Fred]

And was this "edict" based on anything other than ignorance or superstition?

I don't know how thick the sleeving is (has anybody here ever cut it away to find out?), but the starting position is L & N pins with a csa of about 25mm².

Whatever the cause(s) of overheating plugs and sockets there may be, I very much doubt it's insufficient current carrying capacity of the pins.

The edict was most likely based one the results of the in-house testing in the companies own independent test lab's. At the time the company had plenty of spare cash; the testing was probably an industry year project for their sponsored university students. They probably did cut the pins up to find out how thick the insulation was/is (it's probably thicker now than then, mid '70's).
It may not be completely due to the CSA of the pins but also related to the contact area of the pin and receptacle.

Certainly the company's electricians visited every site and changed all the unsheathed pin plugs.