Morris and Lister were a British company and at the time when they were in business I’d have thought it highly unlikely that they would have been using Unified threads, much more likely to have been a Whitworth form, (BSF or BSW).

It is not at all difficult to check which by offering up a clean undamaged bolt of the correct diameter, the one with the correct thread pitch should enter fully with minimum effort by finger pressure alone. If the threads in the holes are less than perfect then it is possible to offer up a tap to the holes to clean out any lumps, bumps and muck, but a word of warning, particularly to those without an engineering background, use the lightest of pressure because a tap is made to cut metal and it would be all too easy to ruin a thread with the wrong one.

For reference a 3/8″ UNF thread is 24tpi, 3/8″ UNC is 18tpi while a 3/8″ BSF thread is 20tpi and 3/8″ BSW is 16tpi.

The only threads that i have noted in M L mags is 16 tpi there could be others or even inserts. threads in soft metals are often of a coarse pitch as they do not strip so readily. i am no engineer just a metal butcher so take the information as you will.Regards and wishes for a healthy and good new year.