I just found this in a Motor Trend article, the 1 or 2 is indeed a plant shift code....
The date code for the AIR pump is stamped on a pad on the side of the pump housing. The date code for this pump is 11901S.
1. The first three characters, 119, refer to the day of that year, this number is also known as the Julian Calendar Date. The Julian Calendar Date is the 365-day calendar with a continuous count of days in the year. So in 1970, 119 would equal April 29.
2. The fourth character, 0, equals the year. In this case, 1970.
3. The fifth character, 1, is the plant shift code. The 1967 AIR pumps were coded 1 or 2, and most 1968 and later pumps were coded 1. 1 equals first shift and 2 equals second shift.
4. The last character, S, is the configuration code of the pump. All 1967 Corvette AIR pumps are coded P. The 1968 AIR pumps with a pressure relief valve installed in the pump are coded Y. The 1968 AIR non-pressure relief valve pumps were coded Z. Most 1969 and later pumps without a pressure relief valve are coded S.
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