Thanks everyone - I've attached a couple of pictures so you can see the issues. One picture shows the amplifier with an original, fitted pot, a modern replacement pot, and the original switched pot (now removed). The other picture shows three pots, one is the 30mm switched unit and I then have a modern 1M log pot together with an older pot.
1. The modern unit has a smaller nut size than both of the older ones
2. The lug in the original unit, that stops the unit rotating in the chassis, is set further out than the lugs on the other two.
https://1drv.ms/u/s!AEB_A0SO1G6dlG0
https://1drv.ms/u/s!AEB_A0SO1G6dlGw