My grandfather's postal code growing up in the US was simply "6". He did not have a zip code, because they had not been invented yet.

We didn't quite get enough info to determine the particular route until Zip+4 in 1983.

> It'd find it difficult to believe that when the system was originally created, mail delivered by a Nevada post office to a physical location in California had a zip code starting with a 9 rather than an 8.

When the US first implemented postal codes, 100+ cities had a duplicate postal code of "1", which was the first postal zone within that particular city. We've increased the specificity of our postal codes twice since then. (well, three times if you count IMb)

But more to your modern zip code example -- addresses and their 5 digit ZIP codes historically followed the post office together in lock step. (This is why many system have separate "mailing addresses" from "physical addresses", they weren't always the same thing) -- e.g. if you lived in rural town and your mail was delivered from a different town you'd have something like a rural route address and both the municipality and the zip code you'd put on the letter would be for town the post office was in, not the town you lived in.

For example, when I was young and wrote a letter to my grandmother, I wouldn't put her street address on it... because she didn't have one. I'd address it to her rural route number and box number, and I would address it not to the town she lived in, but to the town the post office was in, and the zip code of that post office.

In the case where this was across state lines, your mailing address would be a different state than where you lived.

This is part of the reason why USPS has been converting from RR addresses to the Locatable Address Conversion System. Because it was difficult for first-responders to find homes when people calling didn't have addresses that matched where they actually were.