Then I became aware of the amazing work Greg Gagliano had been doing since the 1990s - the summary of his latest results are here - and that his research stopped just before the Rivera-era. However for this range of amps at least, I reckon it's not just a policy of withholding company-confidential information. It's no criticism of Fender to suggest that they were too busy making great amps to keep records just so some amateur can use them thirty years later.
The first amplifiers made in-house by Fender is the Woodie series, built in 1946 through 1948.
They included the Model 26 Deluxe, the Princeton, and the Professional.
Looking at the chart below we can see that there was a pretty consistent numbering scheme for the vintage tweed Fender Champs.
Champion 800 (green tweed)* 01 to 1000 – 1948-49 Champion 600 5B1 (two tone) 01 to 1300 – 1948-49 1300 to 1700 – 1950 1700 to 5000 – 1951-52 5000 to 5500 – 1953 Champ 5C1, 5D1 (tweed) 5500 to 6600 – 1953 6600 to 8000 – 1954 8000 to 9999 – 1955 Champ 5E1, 5F1 (tweed) C00001 to C00800 – 1955 C00800 to C03100 – 1956 C03100 to C06000 – 1957 C06000 to C08800 – 1958 C08800 to C12500 – 1959 C12500 to C15500 – 1960 C15500 to C16800 – 1961 C17000 to C19000 – 1962 C19000 to C21000 – 1963 C21000 to C23000 – 1964 So we can see that a serial number of C 09556 is pretty close to the lower end for serial numbers for 1959 5F1 tweed Fender Champs.
Then Soren in Denmark started showing serial numbers on his excellent Super Champ website, which is no longer on the web - some of those numbers fell in between some of 'my' PRII numbers.