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The early ones were generally not marked until the change to the 80 psi. After that, they were all marked in order to tell them apart. If it has KS on it then it should be an original King Seeley. They have not reproduced these for many years so the later repo stuff will not have KS on it. You can find NOS and good used 80 psi units on flea-pay but there is always a bit of risk there. Barnfind08 (Southside Obsolete) may have some NOS ones.
I'd say there is a pretty good chance that the one in question is a 50 psi unit. All the 80 psi units were marked as such.