I know this topic is quite old, but I just noticed that I can probably answer it and here's what I found:
I decided one of my ps/2's might have been a model 30 and checked. Sure enough, there is one so I took it apart. The drive it has is plugged in using a standard dual-type floppy ribbon cable, so you should be able to use either a nonstandard slotted drive or a nonstandard drive with the standard nowadays connector. The drive plugged into mine, happens to be the relatively newer plug type (i.e. 34 pins, 17x2). Before getting into all the drive labels, I think it's worthwhile to mention that the drive has no power connector (hence my descriptions of "nonstandard" above) so it must be getting power through the ribbon cable, and the motherboard end of the standard floppy cable is, interestingly, plugged into the low-numbered pins side of the (J12 on my motherboard) connector; there are 6 (3x2) pins on the other side that are unused (two of which, are a bit scrunched over to accommodate the edge of the standard cable's connector).
Now for the drive details. It has "Alps Made in Japan" stamped into the metal on one side and the other side has this printed on a sticker: "DFP723D30B" (most likely the model) and "Rating DC12V 0.47A DC5V 0.16A" and "ALPS ELECTRIC CO.,LTD". On top there's more. It's stamped with "IBM FRU 64F0162" and "IBM ASM 64F1008". It also has a barcode sticker which says "L1P60347175". The remaining sticker on top says "Manufactured for IBM Corporation", "EC NO. 844421", "P/N 72X6112", & "91-17 A03". Lastly, it's got a piece of plastic that's screwed to the bottom to facilitate slide-out removal and snap-in replacement of the drive (but I'm sure it could be made to work without it, if you're handy - and it may be specific to my model 30 for some reason too).
And a quick search for "alps DFP723D30B" returned this page, which seems to say your Mitsubishi model (or "Mitsubishi MF355C-599MQ4") is the same and would work: http://ohlandl.ipv7.net/floppy/floppy.html
. I don't know what that page means about the "ALPS DFP723D12F" (a different model drive) having a 32-pin tape connector, as all my flat connectors are still 34 contacts.
Good luck finding a drive (if you haven't given up by now - or gotten it working on your own)!