He is referring to full pressure oil system. 1954 and 1955 1st series are the first 235 & 261. 1955 2nd series they changed a few things. I know for sure the oil pumps are totally different.
I beieve the 1953 powerglide was also considered in the first series.

The earlier 235 don't count in this classification,they where babbit engines,oil dippers.

correct me if i'm understanding wrong.

1951 3100 Chevrolet
1951 Chevrolet Suburban Carryall
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