They were made by two main companies.
G.T. Price and Fulton. They were available mostly in OD Green and Black. Some Navy versions were also available in grey, however, they are the MX-994 model, which is straight, not the MX-991 that you are looking at. There was also a 3 cell straight version, but I can not remember its number off the top of my head.
They are not overly "rare" so to speak. Most can be had for under $10 bucks. However, I have seen some collectors selling them in the original carboard brown boxes with the shipment date shown for as high as $30.
I probably have around 8 or so of these in the various types. I still have one that my father gave me for Christmas over 20 years ago that is a GT Price. It still works fine. They can take plenty of abuse.
edit: I forgot to add. It is very difficult to recognize one from the 80's to current day (they are still made) and the ones from the 60's / 70's. One quick feature to look for is the slide switch. Does it have two raised side panels that form a "guard" so to speak going down the long sides of the switch? If it DOES, it is not that old. If it does not, it is more in line with the era that you are thinking.
Also, one more thing. I have also heard that only a limited number of flashlights that were issued to Vietnam troops actually made it back. Be cautious if that seller is trying to tell you that it actually saw service.
