OIC said the blind man. Thanks for the info Doug. Looks like I will have to take a closer look and report back.
It was sometimes, but not by the mint. It was done by dealers simply to mark them so they could tell one from the other at a glance. The '55 sets came in both - boxes at first, and flat packs later in the year. As a general rule the flat packs are more expensive.
He asked about the 1955 double mint sets not the proof sets. The 1955 mint sets came in the same type of holder as the 1958 shown in this thread. No plastic.