USO made three 7.62x51mm scopes (M40A3) and three .50BMG scopes (M107) in 2005. These 6 prototype scopes were tested by the USMC and were in competition for the SSDS contract that Premier Reticle won. Premier Reticle submitted their gen 2 duplex mil-dot reticle equipped S&B 3-12x scopes, and they got the USMC contract, not S&B. All of the USMC S&B M8541 scopes were shipped in pieces (down to the individual screws) to Premier Reticle, who then built each scope for the Corps. I'm only aware of 1 USO 7.62x51mm scope that still exists, and 2 USO .50BMG scopes that still exist from this contract submission. So, half of the 6 test scopes are either lost/broken, or in collections somewhere.
I actually have 1 of the 3 USO .50BMG test scopes that was made in 2005 and was in the running to be the Corps new SSDS. The scope is 3.5-15x, it has a BDC elevation knob for the Mk211 ammo (300 yard zero to 1,800 yards, 55MOA of total travel), and it has 10MOA each direction for the windage. I also have the original (not a copy) of the USO paperwork for this scope, notice the customer name in the lower right-hand corner. The provenance doesn't get better than this!
Here's another extraordinary piece of USMC/USO history that has never been shown publicly before (I acquired this scope a few years ago, and the previous owner purchased it directly from the CEO/President of USO John Williams). This is 1 of 3 prototype USO MST-150 scopes that were made to show the Corps that USO could build new MST-150 scopes from scratch. These 3 scopes didn't just show off USO's technical prowess and optical capabilities, the scopes went through a rigorous physical test and evaluation (T&E). Even though the prototype USO MST-150's went through this intense testing, they passed the Corps' high standards. However, even after passing the T&E, the Marines decided that they would save lot of money by just having USO refurbish and convert their 7.62x51mm MST-100's to .50BMG MST-150's. Since the Marines went with the refurbished and converted option, no more USO MST-150's were ever made! This is the highest serial number MST-150 ever made, serial number 5999, and the very last MST-150 ever made!
This scope and document is important in documenting what the Corps differ with their .50BMG scopes for their Barrett rifles. There's a lot of speculation as to what .50BMG Unertl scopes were first issued on the Barrett M82A1's, but the history is now known. The USMC Gulf War M82A1 rifles used Unertl MST-100's that were taken off M40A1's (or possibly provided as new scopes by Unertl). That's right, the Gulf War issued scopes that were used on the .50BMG M82A1 rifles were in fact 7.62x51 BDC MST-100 scopes!
This is a fact that has been established: by talking to the Marines who were issued these rifles, and talking to Marines (after the Gulf War) about how they worked with the Scout Sniper school houses to determine the Mk211 .50BMG dope that could be used as a BDC for the Unertl MST-150 scopes that would be made years later in the mid-1990's. When the Corps needed more of these scopes, USO had already taken over the contract for the MST-100/MST-150, and made the 3 prototype MST-150's to show the Corps their capabilities. Instead of spending money on new scopes, the Corps had USO refurbish and modify some of their MST-100 7.62x51mm scopes and turn them into MST-150 .50BMG scopes.
So, the Barrett M82A1 .50BMG rifles were first issued with the 7.62x51 MST-100's, then with .50BMG MST-150's, then finally with 7.62x51 MST-100's that were converted by USO to .50BMG MST-150's. Many people assume (wrongly) that the Gulf War M82A1's were issued with .50BMG MST-150's (either new .50BMG from Unertl or converted 7.62x51 scopes), but we now know that the Marines used 7.62x51 MST-100's during that war.
This is a pretty unique piece of USMC history, and who knows where the other 2 of the 3 scopes are right now. Maybe in collections somewhere? Maybe the Corps kept them? Maybe the CMP sold them? I have no idea.
And for the previous discussion about the USO patent information being engraved on turrets, here is my original& DRMO USMC M8541 SSDS on my M40A6:
The information in this post should be correct, but I'll defer to
@Skunk for the final word on anything USO related. He's easily the expert in this area!