[G3VKM_Roger]

I think the plug-in base and the terminal may indicate that the equipment that used the crystal could change frequency but not as fast as, say, swapping a 10X type. The 4BA tapped hole in the underside of the base may indicate that the crystal(s) were firmly mounted, so again not a quick job to swap freqs.

The terminal is just that, there is no locking nut or other adjustment to the threaded rod part.

The Trawler Band was often marked on domestic sets, as were aircraft bands but I can't recall seeing those bands on pre-WW2 sets, any domestic set collectors care to comment? I will see if I can find some info on the 1947 Atlantic City Conference when a lot changed as regards frequency allocations. I can recall reading objections by European nations to amateur use of 1750-2000kHz ("Top Band") which might indicate when 2MHz began to be used by ships and 2017kHz in particular.

Roger