I was very fortunate to obtain this from a friend on the weekend, I believe that it is a brass cleaning jag for the P'39 & P'42 muskets & would have been used on the P'51 & altered P'42, issued along with the nipple wrench & seperate spring clamp.
These would have been in use for arms with the "button head" type of ramrod & made obsolete with the introduction of the jag head ramrod with the P'53, which came in with the new nipple wrench. Though this does fit down the bore of the P'53 I don't believe that it was issued for the early types with the button ramrod as it is too "snug" a fit. At first I thought it to be a Brunswick jag missing the male thread, but I don't think that is the case.
This is the first I've seen does anyone have one or pictures that I can compare? Thanks, Adrian