Back in the 1960s I decided that I needed an example of a WWI Gew98 Mauser to fill a hole in my collection. At the time, mail order Gew98s in Good to VG condition (without regard for matching numbers) were selling for less that WWII Kar98s so I ordered one by mail.
Considering the purchase price (less than $25), I was pleased with the rifle I received, It was a 1917 Amberg. Most of the numbered parts on the bolt assembly matched; the s/ns on the receiver and barrel matched (but didn't match the number on the bolt) and the s/ns on the trigger guard/magazine assembly didn't match either the bolt or receiver/barrel. The bore was a bit worn and dark but the exterior metal was rust free and had 95% + of its dark blue finish both above and below the wood (which was in great shape with very minor scratches and dings). Overall, the exterior of the rifle appeared to have undergone an arsenal rehab at some point in its history.
Not being that familiar with Gew98s at the time, it was a while before I noticed that the rear sight on my rifle didn't match the "roller coaster" rear sights I would see on WWI dated Gew98s in gun shops and at gun shows.
With a little research I was satisfied that I had what is known as a Weimar rifle that resulted from the German Weimar Republic's modification of WWI Gew98s in the early 1920s. Then I ran across a thread on a forum discussing the Gew98M rifles used by the Wehrmacht and SS prior to and during WWII.
It seems that in 1934 the newly established Nazi Government began sending tens of thousands of Gew98s to Mauser and other factories to be fitted with new rear sights made by Mauser marked S/42 and K for 1934 or G for 1935 (The rear sight on my rifle is marked S/42G). These new rear sights were required for use of the now standard "S" heavy ball ammo and were identical in use to the sights on the soon to be distributed Kar98k rifle. The modified rifles have been designated the Gew98M.
According to what I've been able to find out, the Gew98Ms were the main battle rifle of the Wehrmacht from 1934 till 1935 when the production of the Kar98k reached an acceptable level for distribution. By 1939, the SS was the major fighting force equipped with Gew98M as "hand-me-down" weapons since they were a political organization and not considered part of the Wehrmacht. The SS continued using Gew98Ms right up to 1945 even though by 1943 they had access to all Wehrmacht stores and equipment. Gew98Ms were used and left in Spain during the Spanish Civil War and apparently provided a major source for imports to the U.S. and Australia during the 1960s.