US Optics received the contract for repair and refurbishment of USMC M40 scopes. This meant that all cracked or discolored optics, rusted parts, knobs, etc were to be replaced or repaired as required. USO reviewed the design and upgraded the scope using modern materials and design concepts. Note that the old scopes had been in the field since just after Vietnam and needed work.
My understanding is that the Optics replaced in the scopes were upgraded from the old design and that the erector tube mechanisms were designed to be compatible with the requirements of the M40A3 (MST-100 Scope) and the USMC .50 Caliber weapons systems (MST-150 Scope)
These are the ones (MST-100) that are available to the PUBLIC.
UNERTL has and is building the original UNERTL 10X design and is a faithful reproduction of the original one.
Both sell for around $2500.
The USO version shown on M40A3 clone: