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285642 would be a Rola speaker made in the 42 week of 19x6 (you have to figure out which decade using other information).
For additional speaker manufacturer codes, see Ted Weber's listing at Weber Speakers.
Some of the component parts of Hammonds and Leslies have EIA codes printed on them.
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.