Most of the tone generator wax and Mylar capacitors do, as well as pre- and power amplifier multi-section can electrolytic capacitors.
The code typically takes the form of YYWW, where YY is the last two digits of the production year and WW is the production week. All of the speakers in Hammond organs, Hammond Tone Cabinets and Leslie cabinets dating from 1946 forward that I have seen have production date codes stamped on them. The coding scheme is XXXYWW where XXX is the company EIA code (Jensen = 220, Rola = 285, Heppner = 575), Y is the last digit of the production year, and WW is the production week.
Even the above is fraught with some risk, as replacement parts used for repair or modification will occasionally skew the overall picture.
Additionally, some organs have been pieced together in order to get one working unit out of many; this will produce inconsistent and unlikely date code ranges.
Later Leslie motors have a sticker on them indicating their manufacturing date.
The formula is XXX-YYWW, where XXX is the manufacturer EIA code (190 indicates General Instrument in the present example), YY is the last two digits of the year of manufacture, and WW is the week of manufacture.
In the example shown, 190-7215 was made in the 15th week of 1972.
Earlier Leslie motors have a stamped four digit code.
Until this work is published, the Age List will continue to function more or less as it has; a quasi-definitive source of this information and a registry of Hammond and Leslie product owners.
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.
Recently, the production data was found and there is no longer any need for guessing.
This information is being compiled, organized and edited into a soon to be released book that details all sorts of production and manufacturing information, including tables showing serial numbers and their date of production.