Good score! I have 5cm and 10.5cm tick marks, both from sellers who didn't know what they were so they were cheap, but not were that low. Both have seen plenty of use, but still in good shape and perfectly functional.
Getting back to the 3.5cm f/3.5, my figures above assume the serial numbers are continuous, when in fact there may be gaps and I guess many have been lost or destroyed over the years, so they may be rarer after all. Many were also made in M39 mount for Leica cameras so the number available for Contax or Nikon rangefinders will be even less.