Inside each Leslie produced after 1956, on the lower baffle, is a date code (sometimes hidden by the bass speaker).
Here's how it works: You have to be able to date the Leslie within 10 years, which should be fairly easy.
In the example shown, 190-7215 was made in the 15th week of 1972.
Earlier Leslie motors have a stamped four digit code.
To help determine the manufacturing date of Hammond organs and Leslie speakers.