Here is a interesting street light! There are 2 of these in this post office parking lot and this is one of them. There is also one GE M400 that is 250watt MH. I don't know any thing about this fixture but it could be MV?
Most definetly this is mercury vapor. Clamshell ballasts are difficult to replace (at least in mt OV10 it would be) because the ballast is long and flat.
No not really, your clamshell OV-10 ballast is exactly the same as my MO-8 and my Westy RMA NEMA head ballast....exactly the same, even my Westinghouse OV-15 from 1968 has a flatter ballast but it's a bit bigger since it's it is 250 watts!!!
@ Jace; no ballasts today are the same size though. i do suppose with some metal shop skills one can make a bracket to accomodate the two holes in the housign for the ballast clips.