Any suggestions on how to mark my products so they can be identified?

Indelible, invisible, ultra-violet activated ink is indeed an alternative. But there is one issue, based on my personal experience. Some solvents used in cleaning firearms (assuming they're also use on pellet guns), will remove the ink over time. The stuff I used to use, came from Allied Ink. Others may be available which are less prone to removal. 
 
If it is possible...
Put the ID stamp on the inside of the end cap. That way it is hidden unless needed. 
You might also consider stamping around the face of the area that screws onto the rifle. This would also be hidden until the LDC was removed. 

Edit: You should have made this a contest and the winning suggestion gets a free LDC! LOL! Just kidding! ;)
 
How is putting the stamp on the inside of the end cap going to help identify it, my LDC's are made so they do not come apart so the marking would be totally invisible. Also stamping on a curved surface at the rear of the LDC is not going to be easy, I think stamping the face of the end cap makes sense and I might even use some white or red paint to fill the stamp for more effect, Thanks for the suggestions but for right now stamping the end cap makes the most sense, all the best, Neil.
 
Hey guys I am not going to spend a fortune trademarking something, I just want to make these easier for customers to identify as mine. I do not think there is anyone out there making these who is going to want to try and copy them, the benefits would be what? The people making these already have their own customers, this is just a measure so someone buying one can see it is mine when the seller claims it is a Clague LDC. I am sure there is no real market for someone to start making them and copying my logos,Neil.
 
Neil,
I just installed a HUMA regulator and it came with a simple decal. I put it on my rifle! Yes, I know it can be peeled off but the cost would be extremely low and you could get a clear plastic backing with your logo on it (which won't ruin with water or oil). Look around we all have branded apparell. I would assume cost will be a determining factor. I would recommend it to be small so it can be noticed but not stand out. Falcon used a clear decal on their rifles for example. If it is the size of HUMA's decal you could get about 100 on each sheet. The 1 time set-up fee might cost you more than the labels. Set-up fees from my local printer run about $50-100.00. On-line you could probably get a packaged deal for set-up and set amount of prints.
Hope this helps,

Doc
 
Personally, I like the transfer idea. Something simple on the end cap like . The entire unit could be powder coated (PC) then the transfer applied followed by a satin clear coat to match the PC.

I also like diesels idea of the logo contrasted in different color PC, however as I have only ever PC'ed in one color, I am not sure whether A. the vinyl stencil would survive the oven or B. removing the stencil would cause the powder to drift if removed before the oven.