OP here. Thanks for reviving the thread - there's some great info here! (my R10 drivers are definitely 4th generation - they don't have a stamp, and they have the black felt damping on the back shown in Purk's 1st photo).
An update to my problem: I ended up putting CD3k drivers in them for now, while I figure out what else I will do with these. They sound fantastic, but I've never heard a working pair of original R10s so I don't know how they might compare.
Interesting notes, though: while switching out the drivers, curiosity got the best of me and I listened to the phones with one R10 driver and one CD3K driver. It was a brief test with a sub-par setup, but there was nothing obviously mis-matched about the two drivers.
Also, despite the different specs on the CD3K and R10 drivers (32 vs 40 Ohms), they tested almost identically (33.9 vs 34.1).
I certainly don't doubt that they are different drivers (they have different housing, after all), but my thoroughly un-scientific investigation has at least helped convince me that putting CD3K drivers in the R10 isn't unwarranted. I may not be experiencing the full R10 magic, but what I am listening to is quite nice indeed.