The various changes in siderail designations are simple, yet can be complicated by interwar inconsistency (caused by circumstance, technically Germany was allowed only one manufacturer for rifles, and technically the ordnance depots were illegal, but as much as the IAMCC tried to curtail the illegal activity everything hinged on their oversights resolve - governors, which being political and multinational meant it was always irresolute... just because England, France, Belgium and Italy, etc.. won the war, there was no way around the fact Germany was economically the most important country in continental Europe and the subjugation of Germany, especially economically, was not a universal desire, especially of England).
Generally, the Gew.98 (Gewehr 98) designation is on all G98 receivers 1898-1919, this is fairly static, in the interwar period (Gew.98's are always Gew.98's unless they are modified to 98b or 98k 1898-1945, there was no Gew.98m technically, the Germans of the period defined the upgraded Gewehr98 as Gewehr98 with sS-modified sights, but this was never an on official designation. Gew.98M is a collector designation, attributed to Mark Wieringa's KCN article and used only as a tool for collector distinctions, which is why Mike and Bruce didn't use Gew.98M in their book, they asked my opinion at the time and we all agreed Gew.98M would be inappropriate for a professional books designation), but beginning in 1920 the development of the 98b began, this was separate from the G98 (Gewehr98 are Gewehr98, upgraded or not up to 1945, unless altered to another variation), at first the designation was Gewehr für Radfahrer, then in 1924, when Simson started to manufacture new rifles the official name changed slightly to Gewehr 98 für Berittene (bicyclists to rider), but in actual application the "Gew.98" siderail remained the same.
The official change developing the Karab.98b was ordered November 4, 1925, this is when the formal name was created, - this is the time frame when the official designations Gew.98, 98a and 98b came about, - there was never a Kar.98AZ except at the trials, to the Germans prior to 1925 the designation was Karabiner 98 and Gewehr 98, but for our purposes as collectors, who have to distinguish the variations for discussion, we use the 98a designation adopted in November 1925.
For actual changes on the rifles observed it is more complicated as apparently these changed took time to stabilize, generally all dated Simson 98b will use Gew.98 siderails, this includes the 1926 dated rifles. The various changes afterward define the types of 98b and most researchers use their own patterns, mine are broken into 5 different variations after the dated end, the changes are often influenced by the siderail changes (but mostly the style of the "S" on the RR), which go blank in the early a-block, then in the mid-late a-block adopt the karab.98b, this remains true until the last changes in the mid-late f-block when the August 1930 regulation orders all designations Mod.98. There are exceptions in ranges where the karab.98b is not used, these are probably older receivers cycled through, possibly flawed in some way, but the 98b have many oddities within ranges, much like most maker-dates 1898-1945 that only interest specialists in those variation.
The kar.98k follows the general pattern, most use Mod.98, but most makers have variation to this rule, SDP notably, but others also. Generally if the rifle is a 98k it will use Mod.98 or be blank, depending on maker-date, or if it is a 98k variation, like the 29/40 (Polish components) they follow their characteristics.
This is a general outline only, it can be much more complicated depending maker and period, Simson 98b is anything but cookie cutter production, production of which passed through some of Germany's most trying times, up to 1927 Germany was under significant pressure from the IAMCC (and partial occupation - which didn't end completely until 1935, when the Saar was returned to Germany, really it could be argued that until the German Army reoccupied the Rhineland Germany's sovereignty was being violated, this well into Hitler's regime...), and of course after 1930 (1931) Germany was a madhouse, - without more loans a Hitler was inevitable, or a military state, but the republic was dead and the French government(s) were to blame as much as anyone else (and Hoover said as much in 1931...)
