The M1-C had a Griffin & Howe side scope mount. This required taking a machined finished receiver, sending it to G&H and having them drill the receiver to take the scope mount base. Three screws and two pins were used to keep the scope mount base affixed to the receiver. Said receiver was then returned to the Armory where the mount was removed and the receiver heat treated (easier to drill when the metal is soft). The mount was then affixed to the receiver and ready to be scoped. The scope itself was attached to a scope mount which had a matching dovetail to the scope mount base. Two thumbscrews would be turned to affix the mount to the base. As it was side mounted, it permitted use of the clip.
The M1-C required precision work to ensure that once the scope was mounted, it was perfectly aligned with the bore. It was expensive and required skilled labor. As a cheaper alternative, the M1-D was developed. This differed in that the receiver was not in any way modified. Instead, the barrel was slightly turned down and a scope mount base slid onto the barrel. This was held in place by a pin. Thus, a scope mount would attach directly to the barrel. A matching mount was then screwed into the base. As the scope mount base mounted directly onto the barrel, the handguard required shortening.
Either solution worked. The M1-C came out late in '45 and the M-1D was authorized and produced starting in 1945, but was too late to serve in WW II. It saw service in Korea & Viet Nam.