This is indeed a very rare WWII factory test rifle that has been identified with a small copper tag and the ubiquitous "Ub" proof marks on various areas of the rifle. The noted "Ub" marking is the factory code noted on several Luger pistols that indicated that the weapon was set aside as "test" weapon and was usually forbidden to leave the factory. The testing use has never been identified, but it could have been for test fitting of parts or machining dimension, proof firing and/or improvements in metal finishing. The small copper identification tag attached to the trigger guard loosely reads: "Exercise caution this gun is factory equipment and removal is strictly prohibited by engineering" with the lower portion of the tag stamped "CE/ J.P.Sauer & Sohns/Suhl/43 6644". In addition, the top breech end of the barrel, the pistol grip area and both sides of the buttstock have been stamped with a "Ub" proof mark to alert factory workers that this rifle has been identified as a factory test weapon. The top of the receiver is stamped with a single Eagle/37 Waffenamt proof over ce/43, with the receiver side wall having the mid-war script "Mod 98" type markings used by the Sauer factory. The breech end of the barrel is stamped with "Ub/ce/az" with two smaller Eagle/37 Waffenamt proofs. It has the correct mid-production, stamped/milled barrel band and a stamped trigger guard and floor plate. It is fitted with a laminated handguard and stock that, as noted, is also stamped in two places with the "Ub" proof marks. It has all matching numbers on the various parts. It has an unusual matte blued finish on the barrel with a slightly shinier blue finish on the receiver.