The first 99,999 Pythons have no letter in the serial number.
The increase in production in 1964 indicates that the CNC machining process may have been introduced. Toward the end of 1969 the letter "E" was added as a prefix to the serial number. In 1975 the letter "E" was moved to the end of the number, becoming a suffix. In 1978 and later other letters were used.
I bought this 1968 4 inch bbl Python because it has the Royal Blue finish. Unfortunately this was many years ago and before I had an iPhone 11 Max Pro with really good photo taking quality. This is one of the guns I sold when prices sky-rocketed during the Obama years, so it's too late for me to capture that fabulous finish now.