The Model XXX afterfilters were available for many years after 1954, when original production was discontinued. Another explanation of the filter marked 1973 is that it had been part of a bulk purchase years earlier.
I also mark the date on my filter bags and other filters as a point of reference.
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Correction in my last reply: Electrolux USA introduced the multi-ply bags during the run of the Model E-Automatic, so that would have been in either '56 or '57 (not '58).
