Quoting James A. Markevitch, who wrote on Sat, May 03, 2008 at 06:59:44PM -0700 ..
Confusingly,
the 1e manual for /dev/rk0 says its an RK03. I'll have to go
read the DEC manuals, but I would guess that the RK11 drive and interface is
the same, but the RK03 and RK05 removable disk packs were of different
storage capacity.
The controller is an RK11.
The disk drives are RK02, RK03, RK05.
The RK02 is a 1.2MB drive (256B per sector).
The RK03 is a 2.4MB drive (512B per sector).
The RK05 is a 2.4MB drive (512B per sector).
There was also a RK05F which used a fixed platter (in the same
plastic enclosure as the removables if I remember correctly). That was
double capacity, around 5MB.
Wilko