I have a Rollei brochure for the SL66SE and the SL66X. It lists a2X teleconverter. Here's what it says: "Doubles the focal length of a lens. Simultaneously doubles the aperture value and all the stop numbers. For example, it turns a standard f2.8/80mm lens into a long focus f5.6 160mm. All the lens functions remain fully operational when the converter is in use. In the SL66SE, exposure metering and internal flash metering automatically take into account the reduced aperture."
I'd say go ahead, mount the converter on the body and the lens on the converter. Don't forget to adjust exposure by one stop.
Have fun. By the way I have an SL66 and and SL66E.
Good luck
Morry Katz - Lethbridge, Canada