A Wien Air Alaska 737 seating chart.
Before computers made their way to the airline world, seat dupes (accidental duplicate seat assignments) were prevented with a sheet of stickers that included every seat on the airplane.
Agents would place these stickers on a passenger’s ticket jacket. Once the entire sheet was used up, the flight was most certainly full.
At Wien, the number of seats often varied from 28 to 112 since many of their 737s were configurable with passengers or cargo, in what was called a “Combi” version of the baby Boeing. Sections of seating could be added or removed easily the night before. The sticker card below shows lines that depict the different airplane configurations. Seats were only given out up to the line needed for the particular airplane.
I’m not sure the modern day computer equivalent is as effective. A mistake that would have been nearly impossible with the sticker method happened to me recently when I was given a seat while deadheading, only to have a passenger approach me with an identical seat assignment.
But at least I was able to check-in on an iPhone, I guess.