What 3.5 drive will work with my //c?

["Ed Deans" <eadeans@ibm.net>]

I'm thinking of getting a 3.5 disk drive for my //c so I can make disk
images with my Mac and use them with my PC (whew!).  I've got a //c
(original) serial # D4C1WD5 which has had one minor (it was FREE) ROM
upgrade (but I don't believe it added 3.5 drive compatibility).  How can I
check which ROM version I've got without opening the case?  If I look hard
enough I know I've still got the receipt for the ROM upgrade.

I don't remember if I'd have to use the UniDisk 3.5 or the Apple 3.5 (I
think it's the former).

Any help would be much appreciated!

Thanks,
--Ed.