An example is a 20A UMC shell that has been found to actually be the rare 18 Gauge shell.The 20A also exists in greater quantities in the 18 Gauge.
Dates after company name indicates known dates of the company being in business.
An * after the shell represents the shell is generally a brass shotshell, but the headstamp may be found on others)(1898 to 1932.
The contents of the shell is also of great importance to proper identification.
Sometimes the contents are marked, other times not.
From 1866 to 1932 operated as The Winchester Repeating Arms Company until it was purchased by Western in 1932.