BEP wanted to see if by changing the paper they could get circulating currency to last longer. So some of the 1928-A $1 notes were printed on different types of paper. The differences were the relative amounts of linen and cotton paper fibers. The X-B and Y-B groups were on two different experimental papers and the Z-B group was the control printed on the regular paper. The differences are not obvious to the eye. They can only be told apart by the block designators (X-B, Y-B & Z-B) and serial numbers. The notes I'm posting show significant circulation but that was the purpose of the test. They were released in November 1932. 1928-A $1 X-B block on experimental paper. 1928-A $1 Y-B block on experimental paper. 1928-A $1 Z-B block on regular paper. Their values are low (under $100 each). It was just a matter of finding one of each.