My guess it is an original prewar N frame rear sight that had the rear section grooved. I think the white outline blade was available at that time but this may also be a newer replacement.

It would have only been original for a .38/44 Outdoorsman or a .44 HE 2nd or 3rd Model Target; the finish may just be worn or it may have been redone.

Does it have a serial number on the leaf?

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Alan
SWCA LM 2023, SWHF 220