This clearly is when the component was made and not when the bike was made, but unless the component or bike manufacturer had lots of stock lying around in inventory, the date should be a fairly good indication of the year of the bike.At least it would be the earliest date that the bike could have been made.Bicycles from Japan will have Serial Numbers located on the left, rear axle hanger, on the bottom bracket shell or on the lower section of the headlug.The number will have a production month letter in either the first or second position and a production year number in the other (first or second) position. For some Japanese built Schwinns the headbadge will have a 4-digit stamping that represents the assembly date and consists of the ordinal day and the last digit of the year (2456 decodes to the 245th day of 1976 or 1986 -- use decals and components to determine the decade).
These bikes were built to last 100 years, with reasonable care.
The 1970 Super Sports with Huret dropouts seem to have used a two digit year ('C70204' -- March, 1970, 204th frame).
The 1979 Chicago built Le Tour and Super Le Tour models had their Serial Numbers on the left rear axle hanger and began with an 'S', followed by the standard letter/number scheme.
Modern steel rims, cranks, etc are of low quality, because they are aimed at the cheapest possible price point.
From the mid 19th century, well into the 20th, the word "steel" was magic in Britain.