The biggest reason any Lightweight sat unshipped is that WBR himself wanted to keep the early numbers. He started that by holding back the first 25 serial numbered Standard Autos, gave serial number 3 to one of his close employees, and number 7 and 8 to gun writers.
After that he kept "milestone" numbers, like 3000, 4000, 5000 and so on, onward and upward.
In late 1957 with a big move into the new Southport building on the horizon, somebody there decided to rid themselves of most of the two-digit stuff as well as the "milestone" numbers. This is when Ruger's "low serial number program" was officially started. Most of the two-digit and single-digit guns went out at that time, but there were still a few left in the "factory collection" even after that. Gradually those guns were shipped out.
For the Lightweight Single-Six, I wouldn't say there were any large stockpiles of guns that sat there because they weren't popular...but most regular shipments of the Lightweight didn't end until 1959. Then you had a few "special" serial number stragglers that left after that, as well as the guns that had been set aside to be re-worked at a later date (the factory "second" guns of 1964 and 1965).
Chet15