Good luck. I have one, small pic of a Mk4 Mod O in a book by Kevin Dockery, and you can't see anything. The finish was different too, and FWIW, Victor at US Anodizing does a similiar finish called a 'Navy finish' and it's not just on the exterior - it even involves interior parts. The original finish was Kalguard - an early moly finish. I have written everyone I could think of trying to find pics, including the SEAL museum (in FLA?) and just about everyone I could think of, but finally pretty much gave up. I know the info is out there, but I'll be damned if I can find it.
What I never understood was the o-ring on the buffer tube screw (which would have had a hole in it) but the hole in the stock. I know the idea was to drain the water out of the buffer quickly, and I know it was a 1/4" hole, and I assume it was near where the holes are in the buffer tube, close to the receiver, but it may have been behind where the buffer would ordinarily sit, which seems weird to me - wouldn't that let in water as well as let it out? Was it on the top or bottom? Well, I would assume that since the weapon was based on an M16A1, it had a trapdoor stock, which would also fill with water. Was it drilled all the way through? I have no clue. If you just wanted to drain the receiver extension quicker than the weep hole in the screw allowed, you'd likely have it in the top. Did they tape over the trapdoor or provide a drain holeon the bottom as well? I just have no clue. I know the info is out there somewhere, but no one seems anxious to share it, or simply doesn't remember even if they carried one.