I don't think there would be any way to definitively know. I agree with you that weapons for prison guards would not necessarily be new issue P.08's as the highest priority for new pistols would be front line troops. Prison guards would be a much lower priority, IMHO.
I would further speculate that it might easily have been a reissued WW1 P.08, or even a 1920's Weimar pistol pressed into service.
Perhaps someone that knows more about prison camp issue weapons can chime in with better information.
So... acquiring a shooter-grade Luger of almost any time period would serve your purposes, as far as I am concerned. A shooter-grade Luger would typically be refinished, have a few mis-matched serial numbered parts, and or sport an import mark. Other factors could come into play but the ones I noted are probably the 3 most common. FWIW, I have several shooter grade P.08's and they range in price (what I paid) from $950-$1,150. Hope this helps.
-Bob
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