Bill I have a K 98 marked ar 42 and yes it is undergoing some field repairs so to speak.....a lack of stock parts which I need to replace to make it complete and a half way decent shoulder strap too.
Have a wonderful short barrel Fallshirm G 33/40 marked 965 / 1940, stamp 7654, this unit has the metal/aluminum ? butt plate which is screwed in by the left side with 4 straight head screws...

Bill U should be proud of your collection....excellent !

~E