You will actually find these type of stampings all over the car. They are date codes as to when the item was made. Some are month day shift, some are year month day.
For example, an intake manifold, where it could span many years of use, might be 5C07, made the 5th year, March (A = January, etc.) and the 7th day. In your case, it is 10 21 3, October 21, third shift. Those items typically have a short production life (for example, a fender was only good for 1965 1966 models) so the stamping is a tracking in case they have a bad batch, etc.
Should they match: No, but they were close. A car might have 10 21 3 for one side, and 10 26 1 on the other, meaning they were stamped 5 days apart, and it was just how batches fed into the production line. What you do not have is a car that was built 10 15 having a 10 10 1 (good) and a 12 06 2 (December 6th, 2nd shift) Not good. This would indicate that part was replaced sometime in it's life.
ed ay +3112
1968 GT500 Arrest Me Red (The Big Boy Toy)
1966 K Code Vert (The K 'Vert)