They had tapered elliptical poles "back in the day" but this light must have been trasnferred from another pole at some point. Very interesting that they kept it. I've seen Form 400s on tapered elliptical poles before but they were older looking poles. This is a 60s or 70s pole at the oldest. Looks like a HAPCO.
Oh wow. Is this lighting a street or parking lot? If it's the latter, it may have been removed from streetlight service and a private party bought it and the pole, thus the mismatch. At least in my area, a new pole never has an old luminaire transferred onto it.
Same. They'll transfer and old arm and light to a new wood pole but not a metal pole (the only reason metal poles get replaced here anyways is due to knockdowns, in which case the light is toast anyway).
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From the 1940s! 
The Form 109 predates the Form 400, predating the M-400