The first rule of buying a K98 sniper is that it is the most faked collectable rifle there is. If it is for sale, assume it is fake, unless there is really compelling evidence otherwise.
I was trying to sort out the different variations of K98 snipers, and came up empty, so I bought Law's Backbone of the Wehrmacht, Vol. II: Sniper Variations of the German K98k Rifle
. Good book, with a staggering volume of detail.
Here is a general overview from the book:
Short side rail
1937 Some SS used snipers were conversions of G98s, later made by Berlin-Lübecker Maschinenfabrik, Mauser Oberndorf and JP Sauer & Sohn
Low & High turret
1939, High vs low was evidently determined by the availability and variation of commercial optics. Some scopes needed to sit higher to clear the bolt handle. Early examples made by Army Ordinance offices, later produced Mauser Oberndorf and JP Sauer & Sohn
Long side rail
1943, developed by JP Sauer & Sohn with thicker receiver wall, produced primarily by Gustloff-Werke
Single & Double claw
Both introduced in 1943, primarily used by Waffen SS, who made their own sniper rifles.
1945, attempt to standardize on the Zf4 scope originally designed for G43. They didn't get far before the war ended.
Not intended as a sniper rifle, more of a designated marksman's rifle, with a low powered, long eye relief scope mounted above the rear sight with a side rail. They ended up serving as sniper rifles in some cases by necessity, due to a shortage of proper sniper rifles and scopes.
To further confuse things, there are several variations of each along the way.