Could this be the set you're speaking of? I found it in another thread...
http://audiokarma.org/forums/attachm...achmentid=9778
Motorola roundies of the 60's included the TS-907, TS-912, and TS-916 chassis. I think the one above is either the 912 or 916.
Motorola roundies of the 50's are as you mentioned... hard to find. On rare occasion, one pops up on ebay. Those are usually the 19" roundies from 1955 that may or may have not been converted to a 21" set.
I stumbled onto a 1956 model that I had never seen. This would have been their first set to come with the 21" tube and uses the TS-905 chassis. There is an ad for in the Life Magazine week of May 28, 1956. Unfortunately, this set is in poor condition. If I ever get around to restoring it, it will take a long long time.
http://audiokarma.org/forums/attachm...chmentid=10684