The Wood Shop at the Hamilton Wood Type & Printing Museum has patterns for many different fonts. These patterns are usually stored as letter sets (A-Z plus punctuation, numerals, etc) with each pattern in its own drawer. Most of our type patterns are indeed stored that way, in hundreds of drawers in the Wood Shop. 

However, one cabinet contains several drawers consisting mainly of patterns for the capital letter "O".

Since its shape touches all four edges, "O" is the letter used to calibrate the pantograph for the entirety of a given letter set to be cut. The "heft"--or stroke weight--of a cut letter "O" is measured against the "O" pattern to ensure that the cut type is proportionally correct to the pattern.

This ensures that the heft on all the rest of the glyphs in the set will be correct and will print consistently with each other and with other type cut from the same pattern. 

The "cabinet of Os" was kept to ensure consistency when cutting the same fonts over time. 

It's also a great visual index of the various styles of type patterns in Hamilton's collection. 

The pencilled numbers correspond to style numbers in Hamilton's legacy pattern filing system. Some patterns with more than one number were likely obtained as part of a buyout of another company, which would have kept its own numbering system for patterns. In every "O", the story of a unique history.