Externally they all look identical, however the hardness of the switches differs. My preferred one is PN 1391401 dated 1987-03-04. Another one is PN 1396790 dated 1992-06-29. A third one lost its tag, it is similar in feel to the second one. They are all US layout. The softest one is the first one. I do not know whether it had been used a lot, or the spring specs changed from 1987 to 1992.
I wonder, should I customize them, can I expect modern keycaps manufactured by Unicomp to be compatible?