Five-digit zip codes didn’t turn up till the 1960s (and the additional four digits didn’t arrive till the 1980s); neither did the two-letter periodless state abbreviations we’re all now used to. An envelope in the 1950s would not have been addressed to, say: Boston, MA 02128 It would have been, using the postal zone system devised in the 1940s, addressed to: Boston 28, Mass. If your epistolary novel spanning decades requires this sort of stuff, you’d do well to get this sort of stuff right.*10
