These 10X Snipers are NOTHING like the Lyman and Unertl sporting scopes pressed into service during the Vietnam War.
THIS #1045 was purchased with two others on 27JAN82. It was originally configured and most surely attached to an M40A1 at that time.
The Barrett M82A1 was adopted by the Corps in 1990, and Unertl made this same scope for them as well, but marked "USMC 50CAL.", having a number "5" prefix on the serial number, turrets marked to 1800 yards, and the appropriate cam to suit.
Somewhere along the line, either the supply of newly manufactured "5" prefix Unertls were not keeping up with the purchase of M82A1s or there was otherwise somehow a surplus of M40A1 scopes, so some of the M40A1 "1" prefixed scopes were converted to 50 cal scopes by changing the cam and elevation knob, and milling out the "SNIPER" in "USMC SNIPER" and restamping it to read ".50CAL", as seen on this one.
This scope looks to be in good shape, but I don't think the 50 cal. model will see the numbers a standard M40A1 scope would, especially being a conversion. We'll see; maybe a collector with deep pockets happens to be needing a conversion scope in his 10X Sniper collection, LOL!