The Mark I you describe (i.e. with OD handle and brown sheath) is one of the earlier ones... closer to 1976 than 1981.
I have discovered that this is not entirely correct. From what I am finding, the earliest were black w/white stitching. Then they went to brown, followed by black w/black stitching.
I have:
s/n 15xx that is black w/white stitching and not canted. Green handles.
s/n 8xxx that is black w/white stitching and is not canted. Green handles
s/n 11xxx (not original sheath) but is canted. Green handlesl
s/n 12xxx that is black w/white stitching and is not canted. Green handles.
s/n 20xxx brown w/white sheath and canted. Green handles.
s/n 26xxx brown w/white sheath and not canted. Green handles.
s/n 27xxx brown w/white sheath and not canted. Green handles.
s/n 78xxx black w/black sheath and is not canted. Black handles.
I have discovered an additional sheath variation. This is the sheath from s/n 15xx. If you notice the thumb snap on the retaining strap. It only has one rivet. It is missing the small metal plate as seen on later sheaths which have 2 rivets. Even the (black) sheath on s/n 8xxx has this "release assist".
