"Viet Nam Era" and "75-77" do not go together......

It appears to be a few years earlier than you stated. It has a transition sheath which was punch marked for rivets, and the white stone is consistent with 73-75. I do not believe that it is worth anymore than a standard mid 70's model 18, actually less in my estimation.
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Tattoo Bill
Spring Hill, FL.
Livin On The Edge!
ta2bill@yahoo.com