I bought this one some years ago on eBay, non NSN marked.....I have been told by several collectors I respect tgat the elevation knob is incorrect, asking USMCSGT0331 if correct....
I hate to be the bearer of bad news, but that DT elevation knob is technically incorrect with that ocular housing. The reason is simple, the DT turret was used for less that 2 years, so it was only used on the very first contract scopes. The earliest contract scopes had the NSN on the top of the ocular housing. The scopes went into service sometime in 2006, and the DT elevation turret mechanisms were replaced with the MTC elevation turret mechanism starting in 2007 and into early 2008. The NSN M8541 scopes were all part of the first shipment when Premier Reticle got the USMC SSDS contract. The scope you have will have a higher serial number and the ocular housing doesn't have the NSN, which means that it was part of the last batch of scopes that Premier Reticle assembled right before they closed shop. The Corps was already sending scope in for the DT to MTC turret mechanism swap by the time that they assembled your scope, so it wouldn't make sense for them to put on a DT turret mechanism, just to have the mechanism immediately replaced by an turret MTC mechanism. The DT mechanism and MTC mechanism are completely different, so it's a full swap, not just moving the elevation turret caps around.
It is possible for your scope to have come from Premier Reticle with the DT turret mechanism and BDC turret cap if your scope was one of the civilian scopes they released for sale. In 2008, right before they went under, Premier Reticle would have only cared about making the USMC scopes with the MTC turret mechanism, they wouldn't have cared which style went out for public sale. So, the elevation turret cap could be original to that scope, if it was always a civilian scope, or someone could have put the BDC turret on the scope at a later date. Remember, the DT turret caps were an S&B product, but it was Premier Reticle engraving the M118LR and Mk211 BDC information on the S&B turrets. Premier Reticle also supplied the Corps with spare DT BDC elevation turret caps, which have their own box with label on it. If you find one of these loose DT BDC elevation turret caps, then it’s very quick and easy to swap it out with the scope's original non-BDC DT turret cap.
Just so people are unformed about the turret cap markings, the turret cap is engraved "PR30M118LR." The "PR" is for Premier Reticle, the "30" is the caliber, and the "M118LR" is the specific military ammo that the BDC is calibrated for. If I recall, there were only 2 main shipments of M8541 scopes from S&B to Premier Reticle. The first shipment was in 2006, and the last one was in 2008. All of the first scopes had the NSN engraving on the top of the ocular housing, the later scopes didn't. Premier Reticle actually won the USMC M8541 contract, not S&B. So, S&B had to send the M8541 scope to the US in pieces, and then Premier Reticle built all of them in house. There weren't even sub-assemblies already put together for Premier Reticle. Every single part of the M8541 was loose, all the way down to the individual screw. Premier Reticle build all of the scopes from each individual piece, and they may or may not have used some of those S&B proprietary information when building their Heritage scopes.
The DT BDC elevation turret cap is technically incorrect for your later M8541 scope that doesn't have the NSN and is in a higher serial number range. However, if it was originally sold as a civilian scope, it might have had that DT turret from the start. Or it could be a turret cap that someone added at a later date. I actually own a civilian S&B scope that has no USMC markings on it (straight up regular civilian scope through and through), but it had a Premier Reticle USMC DT Mk211 .50BMG marked BDC on it! The previous owner knew nothing about it, and I have no idea how that elevation turret cap got to that scope. The only rational answer is that a previous owner was a military clone builder or collector, and that person put a loose DT BDC turret cap on the pure civilian scope. So, it's very understandable if a previous owner swapper out a regular DT turret cap for the BDC cap. There is also the extremely tiny chance that some of those DT turret mechanisms were put onto the last batch of non-NSN marked M8541 scopes, but this is a long shot (since they were converting the USMC owned scopes from DT to MTC by the time the parts for your scope were even sent to the US).
I personally think that your scope is a civilian purchase from Premier Reticle when they had a small overrun of the M8541 scopes. They would have assembled the civilian scopes with either DT or MTC mechanisms, so they just used whatever wasn't needed for the USMC scopes that were in the shop. That's just the most likely scenario. No matter what, the other guys commenting in this thread are correct, that is a very rare scope! With or without the NSN, any USMC M8541 marked scopes with a DT elevation turret mechanism are very rare! In fact, I've seen a handful of these later non-NSN DT turret M8541 scopes, but only 1 or 2 early NSN DT turret M8541 scopes. So, the specific type of DT turret scope that you own is very rare, but it's much more common than the other type of DT turret scope. Do to the rarity of being an actual M8541 DT scope (even without the NSN), I'd value it to be worth about $7,000+. It's a very nice scope, and even though it might not be "technically" correct (I'm sure the Corps had at least one of this type just to have an exception to the rule and prove me wrong, lol), it's damn correct enough and rare enough to be an amazing scope for any collection! Put that scope on an circa 2006 to 2008 M40A3, it will be perfect for that clone! Don't ever get hung up about not having an NSN on the ocular housing, it's a great scope!