"When I lived in Boulder, I remember when you could address a letter to someone in Boulder as Name, Street address, and the word "City."
I lived in a small town in Central Pennsylvania growing up, about 2,500 people, with another couple thousand "out the valley" with the same town name/zipcode.
The local post office had two collection boxes, one of which was labeled "Marysville Only Delivery" or something like that.
It still had to be stamped, but you could pretty much use just about any address format you wanted...
Stuff would be delivered if it had Library, Mayor, Moose or something similar for one of the businesses. Didn't need to include the street address or zip code.
If you wanted it to go to a person, you could write the first name and street address, or the last name and just the street.
Or, in my family's case, we were the only Irwins in town, so stuff would get to us with just Irwin. It was invariably for my Dad, who was a Civil Engineer and did a lot of work in town.
You could also send stuff addressed like that from your house by having the postal carrier pick it up. He'd scan it and if he saw that it was local delivery it went into a separate section of his mail bag.
Pretty sure that they don't do that anymore. Town has grown a lot since I was a kid.
The Post Master had started as a rural mail deliverer in town as a teen in the late 1920s. Went away for service in the military during WW II, came back and went right back to work for the post office and eventually became post master. The guy knew everyone in town. Finally retired in the middle 1980s.