Remarks at the bottom of the Cisco DB60 to DB60 pinout page should read...
This was deduced from two X.21 cables hooked back to back. For DCE side I had to add 2 extra wires (Pins 25,26) not used in X.21, and swapped most pin pairs. I currently use this cable between a Cisco 2520 and a Cisco 1601 at 1.5 Mb/sec.
I am currently making a Cisco DB-60 to DB-60 cable using the pinouts of two V35 cables back to back. It should be good for 4 Mb/s.