[Arran]

I sort of figured that your set may have been from the 1940s, I have a model 231 and it's from around 1937-38 vintage so if they were consistent then the model numbers would go higher as new models came out. In which case since it's a late 1940s set then the original filter cap was probably a multi section twist lock can with the can grounded to the chassis, by the way the set should also have a filter choke but if it were open the set would not play at all.
Regards
Arran