250 watt HPS. I have an identical late 80s-early 90s luminaire in my collection.

Lots of room in there.

Has to be in order to accommodate a 400 watt HPS ballast.

Yeah but I think Mike is saying there's lots of room in there, compared to other medium fixtures like the AEL 125.

Yep. Or the OVX (even though it's a small fixture, they cram the 400W gear in there). I find even the M-400 split doors to be more crammed than these. Probably a combination of the split doors having bulkier ballasts and the framework for the refractor door hogging up space. The M-400R2 has plenty of room for a 1000W ballast even!

The other fixture with that much room is the OVM

Yeah I'd love to get both an M-400R2 and an OVM.

I have both. in fact I have TWO M400R2s in my collection. One a 250 watt HPS and the other a 400 watt MH/MV

Those M-400R2s are huge, but I guess without the glass they'd be pretty flat and easier to store. I have a M-400A2 from 1997. Originally 400W HPS 277V but it was reballasted with a 320W 120-277V PSMH ballast while in service. I repainted the housing, added a PC socket, and repainted the rusty ballast core with black paint to make it look better. I had a red "32" tag custom made with the period-correct narrow font GE used at that time (though 320W PSMH was never offered in the M-400A2/R2 line as far as I know, I still wanted the NEMA tag to otherwise be period-correct). Here's a picture of the NEMA tag.