This sticker is a chart that shows the types of tube types used and the order of their physical location in the amplifier, and may be rubber-stamped with two-letter date codes that denote the year (first letter) and month (second letter) of production.
These stamped letters may appear in various positions on the chart.
They are detailed reference resources with a wealth of information on determining the production years of various amplifiers and on Fender history in general.
Indeed, we use these same books here at Fender when researching historical and date-related issues.
Greg and Devin's experience meshed well with mine since I'm essentially the blackface/silverface amp guy (amps made between 19) in the group.