Well, they have this big dice....
What in particular do you want to know. Your question is a tad wide in scope.
For HP41's (and many others) the serial number consists of three parts: the date code, the country of manufacture, and the serial number within the date and country.
for example:
2546S21xxx
is a calculator manufactured in 1985 (1970 + 25) in week 38 (46 - 8 -- apparantly the 8 weeks was to allow for delay between manufacture and retail), in Singapore (S), and was the 21xxxth that week.
This is from memory, I'm sure that if I've got the figures wrong then someone will kindly correct me.
Note that this does not necessarily apply to all calculators (e.g. my HP48SX that has a serial number 3308S02xxx, which seems to indicate it was made in 2003).