what kind of lightbulbs is this for?

Whoa lots of dead bugs in that cool ballast

I guess this is where the computer word "software bug" come from.

This is a HPS 70Watts ballast.

Lovely ballast, looks like German designed, using Siemen polycarbonates as well as TO3 hermetic packages on power devices. Just replace the 2 22uF 400V caps probably will have it running for another 30 years. Looks like a well designed item ,generous ratings on components ( no charred SRBP,only acres of FR22 looking green) with decent filtering and surge protection.

They used them in the 80s years and are working still now. These ballasts composed of big parts are very resilients.