[Choose Ye This Day]

1) When did the Post Office start using the 5-digit ZIP code system? Was that in widepread use in 1972? (It may have been; I don't know.)

2) Would 5000 Longmont have been in a 77027 ZIP code in 1972? Or do ZIP codes sometimes shift boundaries? Did the 77027 ZIP code even exist in 1972?

[Choose Ye This Day]

Sort of answered my own first question. ZIP codes started in 1963.

[Paleo Conservative]

2) Would 5000 Longmont have been in a 77027 ZIP code in 1972? Or do ZIP codes sometimes shift boundaries? Did the 77027 ZIP code even exist in 1972?

A ZIP code is assigned for each post office in the US. As population and economic activity in an area increases more post offices are built. The particular area in question had considerable growth in the early 1970s. It is just south of the mall called "The Galleria" which is the highest end mall in Houston. It is quite liklely the ZIP code changed.