classic! What are those humps where I'm used to seeing a photocell?

Those are capacitor fins. Up until sometime in the 1960s, the capacitors were too large to fit inside the housing so if the ballast had capacitors (which most did; only the "HX" NPF type didn't use capacitors but NPF ballasts for such high wattages are rare) one of these blocks was bolted onto the light to house the caps. I suppose it also helped keep the caps cool, since the early ones might have been more heat-sensitive. Certainly doesn't hurt to keep them cooler.

It was also common back then to have multiple capacitors instead of a single one like in modern lights.

I believe most older 400w MV OV-25s had two capacitors and the 1kW MV B2213 had 4 capacitors. It would be cool to see how the capacitors were mounted in a older finned light.