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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.All available information must be taken as a whole, and, combined with some general tips and observations provided below, a narrow range of probable production years can be arrived at which in many cases will be accurate enough for most people's desires.One thing to remember in this chronological quest is these instruments were built to be played, not (in most cases) to become museum pieces.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.
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.
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.
Tubes with "Hammond" silk screened on them typically have a code consisting of two pairs of numbers separated by a dash.