I have this Toshiba 250w MV lamp that's the shape of a BT37 but is the size of a BT28.
Does anyone know the actual designation for this rather unusual bulb shape or is it just a glorified BT28 with a rounded bulge and long neck?

From what I know, most lamp manufacturers just merely called them BT28 even though the bulge is more rounded and more like that of a BT37 than a traditional BT-28.

EYE/Iwasaki 175w and 250w BT28 lamps seemed to use a similar shape too along with the older Abco-Westinghouse branded 175w MV lamps. Abco-Westinghouse later switched to a more traditional BT-28 envelope.