If you go to [rocksoff.org] and scroll down to the 18th picture (sorry) you see the Stones on Ed Sullivan 1969 with someone on bass who is definitely not Bill Wyman.
Could it be Ian Stewart ?
I had not seen this picture before - it sometime appears cropped to only show Taylor/Watts/Jagger/Richards
I don't have it handy at the moment, but I believe in Stanley Booth's book he mentions that Bill and Astrid had gone out shopping in the afternoon and didn't make it back in time for the rehearsal, hence Stu posing with the Mustang Bass.