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Maybe that valve has the large flange tail?
Was more common on the older TRVs and on the valves with option for copper or iron pipe, which that is.
The photo is difficult to judge, but the more I look at it, the more I am sure that tail nut is the large type.
I agree. It is the older type TRV with a 1/2" female thread on the bottom that you could use with the male nut and a longer than standard olive provided or any other male BSP 1/2" thread.