The first three numbers represent the company that made the speaker, the fourth (and fifth if there are seven numbers) represent the year the part was made, and the last two numbers represent the week that the part was made during that year.
Make note that a source code beginning with 220 was made by Jenson, 328 by Utah, 285 Rola, 137 CTS and 134 by Centralab.
Here is a cool link to a Vintage Fender Amp price history chart that tracks sales data for specific amp models by the month.
Gibson amplifiers have become known amongst guitarists for their warm, consistent sound and reliability.
If it’s a one digit year you need to determine the correct decade of the amp some other way.
ZZ = a number from 1 – 52 indicating the week of the year.
There are several ways to determine when a Fender amp was produced.
First use the cosmetic features to get a range of possible years.
While you may be willing to pay a high price for a vintage Gibson amp, you should always verify the date of the amp to avoid being ripped off.