Dude, I'm building my early gun on a Diemaco as well! A very nice light gray/purple anodizing on it, lots of character. And Gen 3s definitely had the Diemaco uppers so you're double good.
I need to shoot you a PM, I have a ton of pics I've scraped over the years from various forums, Instagram, etc. I always assumed that rifle was an early one because 1) classroom and 2) it has some set screws in the handguard which stick out from all the rest of them. I've seen 3 different variations of that first pattern SPR, ones with no set screws or button screws near the barrel nut/collar, some with 3 button head screws, and some with 4. USMCSGT0331 has one of the VERY early ones with tan Kal-Gard on it that KG Coatings did, while the rest like the one in the pic I posted (your pic apparently) they coated just the handguard in Kal-Gard flat gray, which now is KG Gunkote 2403F. I'll be having mine built up with that coating before it gets spray painted to match a last surplus FSB I managed to coax from PRI. Also they added the button head screws because they seemed to find that when they coated the handguards and baked them, the epoxy that holds the threaded insert in the tube started coming undone. So button heads were added to secure and help keep the whole thing in place. Honestly the ARMS rail is the only thing keeping those early ones together, as trying to get ALL 3, the barrel nut, handguard tube, and then collar to time their gas tube holes together makes for a pretty wonky setup. The Gen 3 basically took a similar approach to KAC and Armalite's tubes by just having two index pins in the handguard tube and sliding it on, then having the collar tighten it to the barrel nut.
And I will tell you, the early guns to get right is nigh impossible anymore without a LOT of fabrication. PRI doesn't have any more of the parts laying around, they've been used over the years for various builds. The barrel nuts just dried up within the last month or so, same as the "Gen 2" handguard collars. I got some of the last old top rails with the lugs for the ARMS rail, and they didn't have any more of the rear shorter ones. No more old side rails either. I'm having a new tube scratch made, and I managed to acquire a Gen 2 rail with an original PEQ top rail. I had the collar put on a lathe by another clone nerd who just put the grooves in it for me.
When it's all done, it's going in a case that came from ODA 535, will have a lower that matches the SN from the inventory sticker that's on it (H&R lower, rollmarked 80% by russianpaint), and hopefully by the time I'm done foam to go with it. Even exploring a custom AEM5 suppressor with the SN off the case to match.
I've got one last part evading me though: the PRI Gasbusted with the "Pat Pend" engraving on it that USMCSGT0331 mentioned above. A few guys have them and nobody's parting with them, and they had a head start looking. It's the last bit I need and off it will go to get NP3 coated like the originals.
Enough jibber jabber though, here's some pics of the project:
Before I sent the handguard parts off:
Handguard collar with grooves added:
Hanguard scratch made tube in progress:
How it's sitting right now waiting for everything to come back: