Unknown 6 pounder headstamp

So gents today I bought this 57x441 casing (6 pounder) because I didn’t recognize the headstamp… It’s two 5s at 12 and 6 o’clock.

Two serif font " 5 "s and a primer adapter, hopefully that rings a bell with someone.

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no-one? starting to feel special :))

The primer pocket & thread arrangement looks very unusual.
Most likely this is something non-standard or an unfinished case or so.
Hard to ID then.