If you go back to the original post of the luger you will note that the luger is from the WW#2 era, in particular an 1940/42 code serial number 20.
In respect to your questions concerning, Roumanian,Polish and Serbian troops I believe these were German conscripts, either you join us and fight or you go to the camps for slave labour.Poland as you may be aware did not invade Germany or make the fatherland it's ally, Poland was basically persecuted in the initial throws of Nazi invasion.
The Hussars units were pre WW#1 units with the totenkopf insignia, after WW#1 the old soldiers that opposed the communist onslaught to Germany formed the freikorps to tackle this communism invasion, many of the Freikorps went on to become the SA stormtroopers. The old totenkopf insignia was carried on to the freikorps by these same soldiers.
Now you bump this up to the Third Reich era and you have the infamous SS with the deathhead on thier caps and badges.
Lugers themselves were regulated just the same as all German ordanance and small arms, the small arms had to be inspected and proofed--ie the stamps of the period eagle 135, eagle 655, eagle/swastika, what the members are saying is that the SS were not above the proofing laws of the day and had to have sidearms officially accepted, SS runes and the totenkopf insignia are not official acceptances/proofs.
Factories that were overun by the Nazi's were utilized by the Germans to supply the growing needs of German troops, the WaA inspectors were installed in these factories to accept or reject the pistols produced in occupied countries, this is why you see Polish Radoms,Hi-powers,CZ's and the like with nazi inspector acceptances.
The fact that luger serial number 20 has had it's acceptances removed and placed further along the receiver causes this particular luger to be suspect, had the booster left the original WaA stamps alone they would have been better off.
You will note that the right hand forward part of an lugers receiver was the official place for the inspectors acceptances, nothing else in the researched documents on lugers stipulates other markings were permissable.
The story coupled with the luger has so many holes that the swiss would be in competition for the finest of cheese.
In order to fully absorb the contents of what is bieng said in this topic it is important that you look at history, the manner in which the Germans accepted their small arms,the earlier totenkopf insignia compared to the one in the photo of the WW#2 holster against the Freikorp photo from WW#1, greed in todays world supported by the price attached to the luger in the Guns/America add.
This obviously does not fully answer your question of other death head marked weapons but it is the lack of them on small arms that begs the question pertaining to the originality of the luger that was referenced.
I hope that this reply has cleared up a bit of your concerns,if possible could you post some photos of the polish troops with the deathhead insignia.