There were early and late versions used from 1948 through 1953 for L-head V8 vehicles (including bonus built trucks). Several manufacturers made the dual action pumps for availability to most auto manufacturers that still used vacuum wipers. The 1949 and early 50 Mercury cars used the first type and the late 50 through 53 used the late type pumps.
Both of my early 51 Mercury cars came with the single action pump due to the low rainfall amounts in the great plains states. Both cars were assembled in St Louis, MO. One was delivered to Clovis, NM and the other to Garden City, KS.