Do you have a picture of it?
I picture would be useful. If it's from Dortmund, I don't seem to have a
picture of it. The 2311 drives I've been able to identify from the pics
were made by are CDC, Century Data, and Memorex.
The 9766 is the drive with the pack on top. There is a CDC FSD drive
directly below that.
Someone at CHM is currently writing a history of the SMD interface for
the mass storage SIG. The interface appears to originate with the 80mb
9762 and 40mb 9760 in 1973.
Formatted capacity can vary on a number of factors, including number of
sectors per track.