I recently purchased an old floodlight and ballast which was being advertised as 700w MV with a clear lamp for $55 which is something I would never turn down but on closer inspection, the lamp turned out to be a GE R400 which is a 400w MH bulb. I was ok with that because I had no 400w MH lamps or gear in my collection and the fixture's pressed aluminium housing was in excellent shape for it's age with no bends or dents. I opened up the box containing the gear and to my surprise, I found a 400w mercury vapour ballast with an ignitor for metal halide lamps in the 175-2000w range. I'm confused as to why someone would connect a metal halide ignitor to a ballast designed for mercury vapour lamps. I've never seen this before and have no idea why someone would do it. I don't think the box containing the gear is original as it looks much newer than the fixture. This makes me think the fixture was indeed 700w MV when it was made but the original gear is long lost and someone paired it up with the gear I got with it.
What I would like to know is
a) Why a MH ignitor has been used with a MV ballast
b) The lamp it should have

I believe it was commonly done in countries using choke style ballasts for MV in order to retrofit them to probe start MH. Technically a MV choke might start a probe start MH lamp when new, but as the lamp ages it'll need the help of a ignitor for it to reliably start.

Over here the practice when converting MV to MH was to swap the entire ballast kit. Typically the probe start MH ballasts had a higher OCC at 300-330v to ensure positive start but were similar enough to the MV ballasts that prior to the MV ballast ban in 2008, probe start MH ballasts were also approved for use with the same wattage MV lamp.

In your case, it could have been 700w MV originally. Using the specs of the lamps sold here, the output of a 400w MH lamp could be considered within the range of the output of a 700w MV lamp for a energy saving retrofit. One way is to look at the actual reflector. 700w MV lamps were usually physically larger than 400w MH lamps. If it was retrofitted, you might find the socket position to centre the arctube on the 700w lamps and the 400w lamps would be lopsided. Now of course, socket positions can be adjusted in some fixtures so it's not a certainty, but if your light has the socket set to correctly align a 700w lamp and not a 400w lamp then it's a high chance it was retrofitted. 

I had a look at the socket and noticed it’s mounted on what looks like an aftermarket plate used for a retrofit. Even with the extra part, the 400w lamp is off centre and looks rather small for the fixture. I’d have to agree with saying it was likely 700w originally and was retrofitted but the seller probably thought it was still in it’s original configuration.
I guess it’s time to start saving up for and trying to find a 700w lamp and ballast so I can restore it to how it was when it was new

In Europe, there are MH lamps for MV gear and MH lamps for HPS gear.