Way back on Thu Aug 20 20:10:13 1992 Mike wrote:

R2 F2 B2 L2 U2 L2 F2 B2 R2  ~  D2 ,
          so that  <U2, F2, R2, L2, B2> = <U2, F2, R2, L2, B2, D2>.

I bet he didn't realize at the time he was finding a minimal
sequence to rotate 2 centres in the super square's group!

If we tack D2 on the end we get the sequence:

R2 F2 B2 L2 U2 L2 F2 B2 R2 D2 =  turn U & D centres 180 degrees
 in 10 face turns.

-> Mark <-