Robbie asked yesterday, regarding 20er rolls:

    If 70mm per second is the "exception," then, pray tell, what is
    the well-defined speed of the "rule"?

See the scale definition at - ahem -
     http://www.mmdigest.com/Gallery/Sounds/mueller3.html 

There is of course a more qualified answer, as each organ manufacturer
has his own idea about this.  But with hand-cranked organs and rolls
without "spiral correction," it is obviously moot to discuss speed
differences of one or two millimeters per second.

Harald Mueller