Nice pictures!
Strange enough, not only there is a visible difference in the scabbard configuration between the USMC and US Army versions, but note also the hyphen on Lan-Cay in the Army version.
There is not many information about these LanCays full tang.
The Brett book shows two of them, both with USMC markings.
Few notes ara available on the Frank Trzaska website.
Here
http://www.usmilitaryknives.com/knife_knotes_2.htm (bottom page)
here
http://www.oregonknifeclub.org/frank3.html (middle page with picture by Cole)
and here
http://www.usmilitaryknives.com/knife_knotes_5.htm (in the middle, with picture of one USMC version).