Definitely an Erfurt rebarrelled with a 6-inch barrel. It would be useful to know if there are any markings at all on the underside of the barrel.
If this is truly a Navy-style barrel (and it from the photo it looks like it probably is) you will find that when you shoot it it will shoot very low. The Navy Luger barrel has a higher sight block, to account for the raised, adjustable rear sight.
I don't think there is any cause for alarm concerning the small frame-rail mark..
--Dwight