The “C” marked on the case-back of a vintage Heuer chronograph indicates that — after the watch was produced — Heuer sold it to its Swiss distributor, “Cuanillon & Co”. Cuanillon & Co. was located in Biel-Bienne.
We are not aware of the purpose of the marking or the meaning of the letters and numbers marked on the cases.
We see different patterns of letters and numbers following the “C”, with the most common being “C XXXX-X”.
Below are some different patterns of letters and numerals following the “C”.