What shell does this adapter fit?

[Depotman]

I've had this fired No. 44 practice plug in a distorted adapter for many years; and I want to know what shell it's from! I always assumed a 6-pr, but an Australian friend has a 6-pr head, without an adapter, with 36mm threads. Unfortunately, my example has 39/40mm threads (impact damage!). Can anyone confirm what head my example fits, perhaps a 6-pr with a larger nose hole?

 

[Depotman]

I've had this fired No. 44 practice plug in a distorted adapter for many years; and I want to know what shell it's from! I always assumed a 6-pr, but an Australian friend has a 6-pr head, without an adapter, with 36mm threads. Unfortunately, my example has 39/40mm threads (impact damage!). Can anyone confirm what head my example fits, perhaps a 6-pr with a larger nose hole?

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I had the usual excellent reply from Norman Bonney, who sent me a scan of the appropriate List of Change - many thanks Norman: