This is a SS Gew98 rework.  Not a lot is known about these guns but here is my take, right or wrong.  

Until the SS was given the responsibility to guard the concentration camps they had no need for military level rifles.  The Heer was very protective of their supply sources and when the SS needed military style rifles to guard the camps, the Heer refused to let them have access to any.  So, the SS set up a rework facility and procured Gew98s from WWI that the Heer didn't want.  

The barrels were replaced with 98k length barrels.  The stocks, if serviceable, were cut down.  Stocks were replaced on those rifles without a funtional stock.  On the reworked stocks, the forward sling swivel was removed with the section of stock that was cut away.  The rear sling swivel was removed, a section of wood was removed and a dove tail piece of wood was inserted.  The stock was also cut to provide for the K98k style of sling.

These reworks typically have death's head markings on the left side of the barrel.   This example has three.  Some are found with one or two.  I do not know if the different numbers of death's heads mean that the rifle was reworked a various number of times or if various operations were performed that required an inspection stamp be placed for each inspection.  There is also a death's head stamped in the pistol grip area of the stock.

Obviously, parts on these rifles were reworked and renumbered.  This example has the original stock that was cut down, the bolt is renumbered as are some other parts.  The bolt release and trigger guard screws are mismatched but may have been considered such small parts that they weren't worth the time to renumber.

Picture Set 1
Picture Set 2