Hi Erwin,
Maybe coincidence? Yes, the 4.5 x 6 back for the SL66 does produce 16 exposures on a roll of 120 film. I finally bought and used one of these backs shortly after my last post and it works as designed: 16 exposures.
I had incorrectly thought that there was a problem getting 16 exposures from this film back, probably because of my misreading some other posts.
I then wondered if a regular 6 x 6 back could be altered to get 16 exposures if one closed off part of the back, thus making the open area 4.5 x 6, while also changing the film transport spacing/sensing. After corresponding with the Rollei people in Germany, I abandoned this idea!
The 4.5 x 6 back works fine....