Some of Stanley’s bench planes had foundry markings on them to identify which foundry the plane was cast in, typically subcontracted by Stanley. This is indicative of the Type 7 planes manufactured from 1893-1899 which has an “S” marking – some speculate this is from the Session Foundry in Boston. On the bench planes it is found on the bed, frog and sometimes the lever cap. The Type 8 Stanley’s (1899-1902) eliminated the “S” castings, but had a “B” casting, which again disappeared by the time the Type 9 appeared in 1902.
So how does this translate to block planes? Well, it gets kind-of muddy here because while Stanley block planes do sometimes contain foundry markings, there is very little information with respect to whether there is a correlation with the bench planes. Foundry markings on most plane bodies are hidden. In bench planes they can sometimes be found in the bed, under the rear handle. In block planes it is harder to hide markings, so they can often be found on the surface of the adjustable mouth plate, where they would be hidden in normal use, and only revealed when the plate is removed from the plane. Here is an example, showing two foundry markings, both “S” and “B” – which doesn’t really give any clarity to the situation.