Andy,
this is a Mauser Parabellum late production.
This gun has been assembled most probably after 1986 using a stocked frame from a commemorative (Artillery, Navy or P08).
At the end of production Mauser started using spare frame, receiver and barrel to create pistol in a "non standard" configuration.
I have in my collection a couple of these late specimens, they are interesting and most probably nice to collect.
I have recently opened a topic about the Versuch model V15 that is is actually the first experiment Mauser made when they decided to create stocked frame with lanyard ring.
The V15 is the unique 29/70 prototype reworked to accept a stock and a lanyard. I guess you will find interesting the topic.
The topic is:
http://forum.lugerforum.com/showthread.php?t=22600
Have fun,
Mauro