The 840 ones look completely different, they are smooth regarding surface, and like Dave says, some of them have the 'new style' BMW lettering ( like you see on receipts / ads even now) written on it.
They look the same as the E38 4-pots ( which fit in no way to our cars BTW)
There are sooo many different calipers being thrown on a big pile here, that its easy to lose track.
The E31 ones will fit E34.
The E31 CSI ones are the same as floater E34 M5
The E31 840 Brembo resemble
the E38 ones (that don't fit!), fit the E34, but different from:
The E32 750iLs ones, which fit the E34 obviously, have the same components as the floater calipers yet carry smaller rotors ( 324 over 345mm).
It appears that there are other brakes for the E34:
1) the brakes used on BT's, Lucas/ Girling cast iron
2) the brakes used on B10 V8 E34's, which are the same as 750 ones if I am correct in my observation.
Mother of all brakes and end of *****ing:
Mov'it big reds!