Hi Steve & welcome to Beoworld.
Not all of the MMC cartridges are compatible only the MMC1 to MMC5 range. The MMC20** & MMC*000 ranges have a totally different fitting (spade as opposed to prongs).
My advice would be to forget ebay & so called "new old stock" B&O original items as these are all aging and tend to suffer from suspension failings. They also tend to attract ludicrous prices for something with no warranty.
The best, and really the only viable option IMO is to buy a SMMC4 from soundsmith in the USA. Myself and other beoworlders have this model and it is much better than the original MMC4 and an upgrade to your MMC5 in any case. This company manufactures these under license from B&O and are the official suppliers to B&O stores - and they come with a 12 month warranty. Not as cosmetically refined as the originals but would be my choice without a shadow of a doubt.
Chris.
The use of metaphors should be avoided like the plague. They're like a red rag to a bull to me.