A stag handled 1989 (M) W77 would have either -
a) come out of the custom shop
b) was rehandled by someone else as a custom job after it was purchased
c) made from unused stock on hand sold off to the public after Camillus bought Western in 1991.
My bet is on "C", as I have seen a ton of non-production Westerns sold via fleabay by sellers from Boulder, Longmont, and other Colorado cities, being listed as "from the estate of a former Western employee" or purchased at the Closing Down sale, or simply listed as being "rare" or "unique", although the knives being listed never darkened the door of any Western assembler.
The last real stag or bone handles Westerns came out in 1960. Real stag handled knives had model numbers beginning with a "5", e.g., the 548. Bone stag handled knives had model numbers beginning with a "6", e.g., the 648.
Starting in 1961, Western dropped all use of stag and bone stag in production knives. "5" was dropped from the handle material identification list and "6" was retained, now representing Delrin plastic as the handle material.
Now, let's add to the W77/777 puzzle
The W77 also came in an all black version in 1986 - "J" year code. Not a 777 model number but "the real deal" W77 stamp.
I have both "M" and an "N" date coded 777s as well as an "M" coded W77. There was also a 757, which was an all black version similar to the smaller W75 boot knife.
So this last knife model number would imply that the handle material designator is the TRAILING "7", not the LEADING numeral as is standard for Western knives. The model number should have been 775 not 757, but I guess the marketing folks liked the symmetry of the 757.
As sac troop mentioned, Camillus made their version - the CP77. There was also a CP75, equivalent to the 757 and W75.
Camillus' CP77 also came as the CP77K, which was a paper model number reference only. The "K" indicated that the knives were shipped with kydex sheaths, rather than leather sheaths.
And to add another misshapened piece to the puzzle, I have a WM77, a Westmark stamped 77, probably made for Smokey Mountain Knife Works by Camillus after SMKW took over the Westmark trademark. No proof, just rampant speculation on my part trying to get all the puzzle pieces to fit together with vague coherency.