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You're correct, until one of the supposed few that were produced turns up lol
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I had one when I worked at Chevrolet Engineering - tagged it for employee purchase the day it arrived in the Company Car Fleet garage; L-72 4-speed SS427, special-ordered by Engineering for Chassis Development. It hit the 3,000-mile mark in December, and I bought it and drove it home just before Christmas; sold it in Flint a couple of years later.
Removed the "Special Order" tag from the left side front fender reinforcement and put it in my rollaway just before I sold it, still have the tag - photo below. There may not have been any L-72 SS427's built on dealer order for retail sale, but there was at least one built for Engineering; I bought several sets of rear tires for it in the two years I owned it.