As with any serial number, the FN numbers give specific information about a product. The FN numbers indicate what year and power type the gun is.
From 1954-1957, the FN numbers are not very accurate. As the production of weapons increased, better records have been kept. In 1964, Browning was using a code T for high-powered weapons, and FN continued using the letter designation. In 1969, Browning switched to a two-digit date code and a four-digit code that identified the type of gun - C being high-powered. In 1977, things became more standardized; the T and C were replaced with the code 245, and a two-letter code for year of manufacture. In 1988, the company redid the serial numbers, replacing the 245 (and 2W5) with 510 (high powered) or 513. The two-letter year code remained the same.