This seller claims that this Klingon "Death Sting" pistol from ST:TMP is real. I spent about 4 hours the other day going over this with both Brett Jones of Federation Surplus (who thought it was a fake) and then Daryl Hyde, a replica prop builder who actually has a cast off an original. Daryl was felt it was a fake as well. Both point to the different paint job as the key indicator.
There is an interesting story about these. They were never seen more than a brief glimpse in a holster in ST: TMP and so almost every one you see out there in kit or built form that is a replica is made from the recasts that were made off the TMP originals. Few people have actually seen an original. You can go to the Art of Star Trek on page 127 and you will see one that was then heavily modified for DS9. They often reused guns and changed them to represent different alien species weapons.
Also, you can see the original below.
You can see the condition the original is in, and you can also see that the paint job isn't even close. What's more is that the seller has refused to answer questions or supply more details on how he claims this is genuine.
REMEMBER: EVERYONE has a "friend" who worked on the show. It is the oldest line in the book when it comes to passing off fake props. And this seller has nothing to back that up.
Now, I will say the holster is cool and this is worth it if you want a nice replica (albeit with a bad paint job).
Finally, check out the Icons reproduction below from the very rare and limited set of Klingon guns they issued some 10 years back.