"...toy like M-1 dummy rifles..." Where'd you see those?
Your 'Old Guard' 3rd Infantry guys at Arlington use modified(note the grip area of the stock) M-14's.
Those Wachbataillon K98's very likely are DP'd rifles. Like the 3rd's M-14's, they make doing the rifle drill easier. It's not the bolt that makes the sound. It's the butt plate on concrete, etc.
"...all of the Nazi-era proof marks..." Supposedly not until 1995(from an unreliable on-line source.). Kind of surprising considering Nazi stuff is illegal in Germany. That stuff was banned by the Allies in 1945. Legally by the new West German government in 1949.
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