Okay, I'm not completely sure what you mean by "glut" whiskey, since most of what I call "blue-collar bourbon" would qualify for that.
OTOH, American whiskey made before the mid- to late '80s was much more flavorful than today's whiskey. Unless, of course, those are flavors that you don't care for; I'm learning that is more often the case than I used to believe.
Your evaluation of the bottling date appears to be pretty accurate. From the late '70s until the mid-'80s, the bottles often showed both metric and ounces measurements; if yours says only "1 liter" then it's probably later than that. The actual federal tax stamp went away with the Reagan de-regulations, but the "idea" of a tax stamp, to show the bottle was unopened, continued for awhile. By the end of the decade even that was pretty much abandoned, except as a cosmetic device. The UPC code came into use in the middle of the '80s. I doubt it was bottled as early as 1980.
It COULD be "older" than 5 years, in that 5 years is only the required age of the YOUNGEST whiskey in the bottle, and that might be only 10% or so, with older whiskey making up the rest. That's not uncommon for high-grade bourbons, but probably doubtful in the case of M&M, which was targeted to a more everyday workingman market. When you open and taste that bottle, though, prepare to be a little amazed at the richness of flavor you'll get from such a non-uppity brand. They all were that way up until the end of the '80s, and I'm certain that -- assuming you appreciate all those extra flavors -- you'll be delighted with your purchase.
If not, PM me and I'll happily negotiate to see if I can relieve you of that swill.