@Ralfsnart What you have in the plio-film pack then in the brown envelope is the Proof-Like (PL) set.
The PL set was initially issued in the Pliofilm pack in 1961 and continues today in the soft, heat sealed package. Over the years, the PL designation was dropped for BU, the envelopes went from plain brown to pre-printed then hard cardboard but the coin wrapping is essentially the same.
The hard plastic holder is an aftermarket holder and someone has broken open a PL set to put it in the more protective holder. Since the hard plastic holder is post mint, you always run the risk that someone has swapped out coins with business strikes or circulated coins.
The original
RCM PL sets were issued from 1954-1960 and were the 6 coins in a white cardboard holder, the Dollar was in the middle, and the whole pack was wrapped in cellophane.
Until 1967 the dime through dollar were silver, from 1968 on, the PL or BU set only contained base metal coins.