[NIB]

It seems odd that all the rounds from the case would not have the same head stamp....

Yes, to the above.

While it is true that the "U" headstamp was once heavily used at Remington (as Rem. was once owned by U.M.C.) one has to wonder if the case of ammo mentioned in the original post was a recent production case of Remington 22lr? What recent Remington production would use the "U" headstamp? I'm seeing mostly if not all "Rem." headstamps?