I don’t know if this is actually the case. British M61 is noticeably lighter than M61 APHE, because as far as I know that weight is just the filler and fuse removed with no concrete filling.
I say “concrete” because I’ve never seen a primary source showing it was done. Seems like a waste of effort and man hours. I’ve found way more information on the British using both inert and APHE M61 than anything suggesting they filled the cavity with concrete.
M61 must have been supplied by the US right?
I think I vaguely remember a document listing 75mm ammunition, including inert M61.
So they most likely were produced that way by the US for Britain.
The inert M61 uses a base plug with tracer. Would be strange to remove the base fuze with tracer, remove the explosive filler and then screw in a different plug.
You could just unscrew the fuze from the whole assembly and would be left with just the tracer.
So unless the shells were produced inert, they wouldn’t have a different base plug.
It’s also interesting to note that the fuze lacks any bore saftey provision.
If something is inside the barrel, the round could detonate inside or shortly outside the gun.
Probably one of the reasons why the British didn’t want to fire these shells.