Long since out of production. SureFire used to offer D2 and D3 models, which were essentially 6P and 9P models with pocket clips. Same P60/P90 (respectively) Lamp Assemblies. Only difference is the addition of the pocket clip. Despite the "Defender" name, they were never produced with crenellated Strike Bezels such as those found on the E2D/6PD, etc.
If it's exactly the same as the one in the photo, then yours is one of the earlier examples - later on the markings changed slightly and the hexagonal anti-roll bezels were used, and they came with the newer Z41 LockOut tailcaps.
It would be interesting if you could post a picture of your light so that we could see the tailcap. If it's a Laser Products tailcap then there's a good chance it's a Z14 rather than a Z41 (LockOut) model. If you plan on using it then I would definitely make sure that it's a LockOut tailcap.
A nice piece of SureFire history, regardless
I would imagine that they were discontinued because they didn't add much to the product line when stood next to the 6P/9P and C2/C3.
As with anything discontinued by SureFire it would probably be fairly sought-after, to the right person.
I hope that helps, let me know if you wanted to know anything in particular
Regards,
Tempest