More: In those days, before personal computers and especially when dealing with important databases like Social Security, every keystroke would be “VERIFIED” by a second individual.
The verifier actually re-entered every item and if there was a mismatch between what the verifier typed and the original key-entry person typed, then the verifier would be notified by the verifying machine and action would be taken to correct any mistake, such as a “typo” OR a misreading of the zip code.
It was next to impossible for such a “typo” to get through, but if it did, then there was another test: The computer program that processed the data to meld it into the computer file would have additional checks built in to find “exceptions”. An “exception report” would list the potential errors and mismatches and that data record would NOT go into the database or the file until the error was corrected.
With exception report in hand, a human being would work to get to the bottom of the mismatch.
An example of an exception, in this case, would be the alphabetic state code (HI) (keyed in for the person’s address) NOT being consistent with the key-entered zip code (a CT number). Computer programs contained tables of items like zip codes, just like what you linked to in your comment. The tables would be used within the program to compare the keyed data to what would be expected based upon data entered in other fields, such as ensuring that a Hawaiian address had a Hawaiian zip code.
In the case of SS#s, it’s very likely that a zip code table was used in the program, specifically because SS#s were prefixed by state. This grouping was also characteristic of early computerized files. It made sorting easier.
Two-operator keystroke verification and computer program exception reporting was standard operating procedure in computer rooms back then, in the same way that CPAs have standard operating procedures for keeping books. In fact, back then, CPAs who acted as auditors also audited the procedures and file-keeping in the computer department, as well as in other departments of a company, such as accounts receivable and payable, inventory control, cost accounting, payroll, etc.
All this makes the scenario you suggest even MORE unlikely.
That’s very possible and I think provides a rational counter argument.