Here is a selection of examples I found on the web... some are a bit scary...

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The top left woofer could be the real thing.
All the others are definitely wrong.
Those in the photo with the speakers on stands look like Beovox 2600 woofers, which are excellent drivers in their own right but
completely unsuitable for this use.

It really takes a look at the magnet side to tell for sure if the driver is correct.

I don't think people replaced the woofers in an attempt to improve anything.
I think they did it, because they weren't aware that woofers with crumbling surrounds can be repaired.

Whatever they fit won't improve anything, anyways. The original woofers are custom produced for B&O and
I have still to see any replacement coming anywhere near.

Martin

Thanks Martin! What about things like diameter of the dust cap, or the material from which the cone or the dust cap is made? Are these known or specified anywhere? My S50 for instance have a soft, "porous" cone - on the pic below (hope that's original...) However, the S120/MC120 on most pictures seem to have a more "solid", shiny cone. 

For instance, I could be fooled by the third picture, if the guy selling them would not admit explicitly that he replaced them with Visaton drivers, and this is what scares me.

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The drivers specifications are not disclosed, but the cone is polypropylene and the
dustcap is some kind of laminated woven thread.
The woofer cones and dustcaps in the third photo are too shiny. The original cones are much more silk-like,
like the left woofer in the first photo.

The dustcap of that woofer could also be correct, though apparently it shows the result of
a rather messy DIY-refoam job. At least is seems to have been shim'ed.

Your last photo looks a bit like Beovox S45/50/55/80 (refoamed but not shim'ed).
This woofer is not correct for MC120.2 either, but the dustcap is correct with the shallow inwards going ridge close to its circumference.

Martin

Here is a photo of what I believe is a refoamed original, and it fits your description - does this one look correct?

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As mentioned earlier, you really need to see the magnetside too to be on the safe side, but this one looks original from the front.

Martin

Thanks a lot Martin! I will keep that photo as a reference. I doubt unfortunately that I'll have a chance to look at the magnet side before pulling the trigger on a purchase, so this will have to do. But it is already incredibly helpful. (If I manage to find a pair they will be my 7th pair of B&O speakers).

And by the way, the soft cone posted earlier is indeed from my Beovox S50, I got a very good deal on them with the original stands and in great condition, although I needed to refoam them. They are now playing in my stereo system and I like them very much, even though they're only the bottom of the 80' S-series.

Piotr

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A good indication is the price and foam rot as that would usually be the reason for getting rid of them. It is not meant as a joke but a fact.

I know with my S45.2 and S120 that was the case. My M150 had the drivers replaced I was told. 

Dillen serviced both pairs in 2008 and they still play great.

I wish this would be the case, but where I live vintage B&O is primarily imported by firms/people specializing in vintage audio equipment, you cannot get it here from a long-time owner or at a yard sale, unfortunately. There was no B&O behind the iron curtain ;) 

So my only options are places like local auction sites or perhaps even ebay. The sellers might swap the driver trying to get a better price for a nice looking speaker, or perhaps not even know themselves the drivers were replaced because they just got them like this elsewhere in Europe, brought them over to sell here, copied the writing on the label into the description and they're done. 

In any case, since the S120 are a bit more expensive I certainly wouldn't want to end up with a modified version ;)