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I'm sure the cups are the same on the recent ones, but the spacer/axle sheath jobby between them is longer on the MTB ones. When parts were impossible to find I fixed a couple of road bikes with MTB cups and the old spacer from their road cups. I thought the cups were slightly different before then but they looked the same, worked fine too.
snottyotter
The difference between Shimano ROAD and MTB bottom brackets is the length of the threaded portion, the sealing and the width of the "outboard" part. MTB bbs are a little bit thinner.
MTB 10.3 mm vs 11.3 mm ROAD.
If you put a 1 mm spacer on each side you can just use the MTB bb cups with your ROAD crankset. :-)