[Southpaw]

It does not appear to be a very delicate tool and is such a beefy looking wrench. It also appears to have been used a good bit showing signs of wear. It also has a broad side that would allow it to reach down in a recessed area. Very strange.

If not as part tool assembly as other mentioned and on outward appearance on gun it is not clear if it was used directly on gun.

Can't tell clearly but there does appear to be a metal butt plate. Could there be something on the stock under that buttplate that would have a nut that size needing servicing. Or a nut that size somewhere hidden behind the sideplates of action. That maybe something unique to that gun but I put it at long odds that a reason for wrench. It also look worn enough and solid enough that it was used for something common.

I can see why this is a stumper for someone even with Bruce's vast experience because its reason for existence just doesn't jump out. The only other thought it, was it was remnant wrench that fit something in tool set that needed that wrench but then over the years changed it something smaller and did not need wrench, but just left it as part of the set. There maybe no easy reason for it or there maybe some unique trick for its use that is eluding.