eljefino has it exactly right.
Prior to 2000, the code was 3 digits, and the arrowhead (triangle) was optional, but widely used, but that really started in the early 1990's, so Dunlop might not have made the transition, if they made it at all!
The transition to 4 digits started late 1999, and completed mid 2000.
DOT codes weren't required on both sides until 2008 - and even then the date code was only required on one side - and it was the side intended out. The other side has to be compete except for the date code.
But prior to 2008, this was all optional. The only thing was that the complete DOT code (including date code) needed to be on one side.
And the first DOT codes were made somewhere in the late 1960's. Prior to that was a manufacturer code of which there were many, many different systems.
Plus, DOT codes are only required on street tires. Other types of tires MIGHT use the same coding structure without the letters "DOT", but many manufacturers do not (to prevent confusion between street and non-street tires.)
Just an FYI, the company I worked for produced street versions of their racing tires, and the racing versions had a really abbreviated coding (plant and date code only), but the coding was the same as the DOT coding.