Hey there Ryan -
The hard part about the Ring Mount system on the LVS (as far as I know) is not so much the M66 Ring Mount itself, which is almost itentical to the system for the HEMTT's, but rather the roof hatch system for the passenger side of the LVS. I believe that these were unique to the Marines' LVS only. Where the HEMTT Roof Adapter was basically a simple round opening in a flat roof plate adapter, the system for the LVS was a sliding panel which slid over to the driver's side for access to the ring mount from inside. Stuart in Washington State could chime in here as I think Damon (who did much of the restoration of the one that Stuart has) actually adapted an LVS unit to the HEMTT cab as Damon (as well as I) think it was a little bit (however more complicated) better system. An open HEMTT system could always be grafted into the removable roof panel on the LVS. When I get mine installed on my M985 and my PLS, I am going to have a removable, 2-piece hardcover (aluminum?) made for them. Originally, the HEMTT's just came with an elastic bordered, OD Vinyl cover that went over the opening.
Dave