The stem broke at the extension or more precisely, the rear clamp cracked. If the bolt was not original, which seems to be the case, maybe that was the cause. According to Chuck, the newer style Cinelli stem with the recessed allen bolt in the back dates to c. 1972 and since this is a '73 Cinelli SC, I guess that's what I need.
And yes, Roman, as I said.. it's my fault for offending the Italian cycle god. I am unworthy of riding, let alone, owning a Cinelli SC. I really should offer her, free, to the first CR List member who asks.
Peter Kohler
Washington DC USA