I believe it is a genuine 1914 military luger, it obviously spent some time in the UK and gained commercial proofing there.
I also believe the Totenkopf to be a later and spurious addition. The Death Head marking opens an entire new or really old discussion as to whether the DH marking is original or fake. There are a number of threads with pictures on this forum discussing the DH lugers.
I found one for you:
http://forum.lugerforum.com/showthread.php?t=39442
This particular DH is orientated in a different direction than (all) others observed or pictured, and is obviously a pantograph addition, and somewhat crude to boot.
It is "safe" to assume that DH markings are fake- as to prove originality would require provenance other than just its presence on the pistol. JMHO
A couple other pictures of DH lugers, note these markings all appear to the stamped, rather than pantographed: