Yep, it's a M-400A then.
Yep, they were made in mercury vapor and offered in MH too but i doubt there were ever any Mh ones sold since at the time MH wasn't really practicle. (really expensive and you'd be lucky to have a lamp make it over 10,000 hours). There should be a lable on the inside of the upper housing and there'e where all the info is like ballast type, voltage, watts, and the date code. The M-400, being HPS and having a NEMA tag, is from the mid 70s to whenever the M-400R2 and M-400A2 came out...
just to let you know, you can't convert a power door witout ruining it so i woudn't do it. My M-250A is the little brother to the M-400A and maybe they made the powr/doors a little more versitile later on, but with mine the plug is permanetly attached to the wires so the only way to change the ballast is to cut the plug off, strip the wires, and find another plug which ruins the originality of the light. Or course it is yours so you can do what ever you want. I'm jsut saying once you modify it in anyway that's it, no going back. the idea was that you'd buy a whole new door instead of trying to change the ballast yourself, yet another GE money making innovation.
250W HPS is bright, but so is your 400W MH highbay. 250W HPS is dimmer than 400W MH. 250W HPS is about equal to 400W MV, which isn't that bright. My M-400 (same as the M-400A but with a top mounted ballast versus it on the door) is 400W MV and while it is bright, it's soothing (well, it's also a coated lamp and i don't light it often. When you try to get a lamp, try to get a coated lamp so it's less harsh on your eyes. My eyes are very sensitive to the light. I have to sometimes wear sunglasses on cloudy days and when it's sunny i squint even with sunglasses on so glare is something that bothers me, which is why i like coated mercs over clear.)
anyways, I'd personally leave it all original but obviously it's up to you. I hope you can get it! i'm guessing it didn't have a lamp when you opened it then? To open the refractor part, the bare metal bar in the front of the lightpulls out away from the fixture and the door will come down. It's a spring loaded latch. To close it, just puch the refractor door and top housing together and the latch will make a lound "snap" as it engages. Also, don't forget to put in a photocell when you try lighting it up. i forgot to do that when i tested my M-400 and I was all worried when it didn't light up lol. Then after 10 minutes i realized i never put in a PC lol.