Anyone tell me what this pistol is please

http://i138.photobucket.com/albums/q...5012009673.jpg

The grip is plastic, front sight is metal, back sight is plastic but quite sturdy, obviously the blueing has been removed.
It has "CAL 4.5/.177 64388" stamped on the breach, "Original" in fancy script on the top, "MOD.6" behind the rear sight, and "Made in Germany" on the left side.
The breach is really loose but it cocks and fires (little bits of rust/muck come out when fired).

Hopefully someone can tell me what it is or at least if it's worth sorting out.

Thanks


Jon.