From 1950 to midway through 1955, the US Mint produced Proof Sets packaged in cardboard boxes. From 1950 to sometime in 1954, each coin was enclosed in a separate cellophane envelope. During the production of the 1954 sets, the US Mint switched from the cellophane to plastic poly bags. Therefore, some original 1954 sets will be packaged in cellophane and some in poly bags. All original 1955 Proof Sets had the coins packaged in poly bags.
Since the cellophane envelopes do not stand up to time very well, US Proof Sets still in the original cellophane and outer box command a significant premium over sets displayed in Capital holders.