I suppose these were all modified military Mauser actions done by the makers - not a factory Mauser option (?)
I know of an English Rigby (SB magnum action) with an extended top tang and was wondering if this could be an original Rigby modification.
If I recall correctly, I have seen this on an H&H built bolt gun once, but it went over the comb as on many of their doubles. This Rigby tang doesn't go that far and doesn't have a safety or anything else on it.