I just realized I made a mistake when describing my sisters machine. It's actually a two speed machine with NO power nozzle outlet. So assuming serial numbers are consecutive, if I list the above machines by serial number(and therefore age) we get the following:
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<p class="MsoNormal">S.N. 0439599 is a two tone finish model 31 with a one speed motor and a power nozzle outlet purchased new in 1975.</p>
<p class="MsoNormal"> </p>
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<p class="MsoNormal">S.N. 8932096 is a two tone finish model 31 with a one speed motor and no power nozzle outlet.</p>
<p class="MsoNormal"> </p>
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<p class="MsoNormal">S.N. X9023784 is a monotone finish model 31 with a two speed motor and no power nozzle outlet.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">So it seems that the power nozzle outlet was an optional feature, but look at the difference in serial numbers between the first machine and the second. That second serial number is 8,492,497 higher than the first. That's eight million, four hundred ninety two thousand, four hundred and ninety seven higher. Is that even possible? I'm having a hard time believing Filter Queen was able to sell that many machines, especially when one considers what they were asking for them. That's a Filter Queen for one in thirty seven people in the U.S. today. Considering the population was smaller in the 1970's when these two machines were sold, the ratio would be even higher. Clearly something else is going on here with the serial numbers, but I have no idea what.</p>