The proof date code is usually comprised of 2 letters inside of a square box that is located on the side of the frame.
Some of the earlier proof date codes are stamped as either a single digit or Roman numerals.
The code does not represent the year of manufacture but rather when the gun was proof tested which is about as close as we can get to determining when a gun was made.
Here is a link to mykeal's proof date code chart:
This reference page shows a photo of a sample proof code box that contains 2 letters: