I talked with an Aerus dealer the other day and was informed that the company's serial numbers were never coded. Furthermore, he told me that the serial number register only provides the date when a cleaner was initially sold—not when it was manufactured (which may be quite different). The customer purchase records that are maintained for follow-ups and service checks—taking for granted that they include the serial numbers—would therefore only be speculative regarding years of manufacture.
The dealer also suspected that the headquarter's old branch inventory lists—which would have provided periods of manufacture—that Jimmy referred to in reply #1 were discarded one of the times the company's headquarters was relocated or maybe when the Old Greenwich factory closed in 1985.
Therefore, the only possibility of obtaining old inventory lists by serial numbers would be if a branch manager/franchisee kept records which haven't been discarded.