In it's day...we are going back to the early to mid 90's that scope was T.H.E. scope. It was one of the best made back then that could stand up to the recoil of a 50 cal. It was also probably one of the first to have really great repeatable turrets. In those days Leupold didn't supply the scope with Mil-Dots so you had to either buy one from or send yours to Premier Reticles. Dick Thomas, the father, ran it then and was in Winchester, Virginia. Now the early ones I am describing didn't quite have the glass they have now. You could run out of shooting light with one of the older ones before you would with the ones they make today. Leupold still does make that scope. The M1 was the target model, with 1/4 minute lick adjustments and came in 10X or 16X. The M2 had a "butterfly" or V ranging reticle and was discontinued soon after it's release...seems Leupold didn't want to be associated with the killing of humans and even today if you call and ask about it they all act like nobody knows what you are talking about. They didn't make many M2's with most purchased by the Dept. of Energy. The M3 was the standard U.S. Army sniper scope on the M-24 for many years, up to the time of the AN/PVS-10. It was also fixed 10X and had 1 minute of angle turrets. All Mark 4 Ultra series scopes are guaranteed for life to the owner whoever he might be at the time, doesn't have to be the original owner. The only scope as tough as a Mark 4 Ultra in the day was the Unertl 10x USMC sniper. I have two of the early 2000's years Mark 4s and the glass has been improved over the first one I bought back in 1991. I understand Leupold is working on Mil turrets that they will retro fit to your Mark 4.
As to current value, they seem to be hanging in there at around $875.00-$1000.00, many did not have mil-dots. Later, Leupold started putting in the dots but they were not the football USMC style. The M1 with it's 1/4 minute clicks is still one hell of a nice tough scope and I have two rifles fitted with them I shoot all the time...wish they were mil/mil but I guess it wont be long. When that happens I think you will see a lot of guys looking for these scopes. The IOR is probably better glass but it aint no tougher.