Here's what I've observed, which may or may not be right. I'm not sure its possible to date from the serial number: at one point I had collected a number of serial numbers of know, dated bikes and there didn't seem to be a pattern.
I think they're easily dated to a period by the details; there were distinct changes in features: braze-ons, holes in lugs, style of headbadge, colors and chrome if original.
I understand at the height of production there were about 350 classic Cinelli's a year, less in Olympic years as a lot of production went there. I'd guess rarity is pretty much like any racing bikes in America: 50s, pretty rare, 60s rare, 70s, not uncommon. I've had 6 or 8 of them over the years and was never particularly looking for them. This will seem politically incorrect, they're nice bikes, but worthy of the worship? Many others ride as well. The 60s ones are pretty sweet from what I've owned, but my mid 80s one rides pretty nice too.
Ted Williams