I guess that is a weird base that looks like skirted base
My guess that the same shape bulb is also used in Intermediate Edison Screw (E17) during manufacturing process. That is also my guess that is how they save money switching one base to another without reshaping the neck of the glass
I suppose so. It also seems a little tricky to make small glass wells for candelabra bases. Also remember some old Japanese decorative bulbs and even a few current dollar Chinese decorative bulbs come with shouldered candelabra bases.
Last Christmas, I was given a clear plastic display case that has 6 different Duro-Test bulbs for demonstration purposes. The thing can be plugged in so they all light up. It has this same size bulb in the unusual 8 watt size. There are clear, "icy" frosted, clear and amer "Spun Glo" (with the filament wrap), blue, and pale pink. I believe it was used by salesmen to sell the products and looks like it is from the '60s or '70s.
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My guess that the same shape bulb is also used in Intermediate Edison Screw (E17) during manufacturing process. That is also my guess that is how they save money switching one base to another without reshaping the neck of the glass