This knife was adopted in 1957. From 1957 to 61 it was made with a 6 inch blade. The blade was shorted to 5 inches due to pilot feedback and the basic design of the knife has stayed that way since.
Here are some examples of the markings on the Camillus made knives.
1958 thru 1966
1967- to the mid 1980s.
Camilluss contract for providing these knives to the U.S. Government ended in 1984. The Knives continued to be made with the pommel markings including the month/year date for a time afterwords. Im running a thread on the site trying to see what the latest dated examples might be.
Mid 1980s to 2006
While these knives wouldnt have been provided as part of a Government contract and issued to active duty servicemen through that procurement process. Camillus continued to supply these knives to the U.S. Government AAFES, (PX/BX) system. So if you run across one of these knives that someone has and they say that they or a friend or family member got it in the service. IMHO I wouldnt challenge it.
I think your pretty safe regarding the quality of the knives through out the manufacturing years of them. The basic steel is a 1095 or similar type steel. This is a high carbon steel and is subject to rusting if not cared for. Just wipe it down from time to time like we should all do with our knives, and you should be fine.
As regards Vietnam correct knives. I would say that any of the 6 inch knives, the first blade marked (CAMILLUS/NY) knives, and the pommel dated knives from 1967 to IMHO maybe 1972 dated knives would be in that category. While the war officially ended in April of 1975. We were not sending new personnel and or equipment to Vietnam in the last few years of the war. Most equipment that has genuine provenance and can be accurately dated will have 1960s dates attached to them.
The knives that fall into the Vietnam era category tend to go for a premium in asking price.
Some general thoughts about the places like the BAY.
The $750 example you mentioned in the OP. Ive seen that knife and would point out that its been sitting there for awhile now with no takers. Its a great example of an early 6 inch knife. One you could say cant be upgraded. The asking price is what the seller is asking for it. Thats a lot different than what its worth.
Youll find that there are sellers in places like this that post items like this all the time. They are waiting for that impulsive buyer that has no patience and more money than sense. IMHO besides the entertainment I get from seeing such asking prices I generally ignore them. Its much better to see what similar items go for in actual auctions. Remember that you must pay attention to the condition of the item. Details make a lot of difference. There really isnt a short cut to this. Its something we learn. Knowledge is power.
As an example go to EBay and do a search for this item number. 390599269601
This is a very similar knife, minus the packaging but its a screw pommel knife, ( this is the earliest version of this knife and usually adds a premium to it). The selling price for this is IMHO a bargain considering what it is and its exceptional condition. The point is if you know what your looking for and the market there are deals on these places while some refuse to do business because of some of the sellers to be found there.
The price range on these knives are all over the place.
Hope this helps.