Aside from sending in a newly created unstruck planchet within the very first year it is issued, another way a specific year can be determined is if you sent an unopened Mint-sealed bag of coins to a grading company to be certified. Read the label on this slab below concerning an unstruck Peace Dollar. (Not mine)
Don't you see it? You have to tilt the slab to the left to read it normally. Why didn't NGC place the Blank so the writing could be read right side up? It's tilted to the right!
Wow. Totally missed that! I also have a few errors where they slabbed it with the prong covering up part of the date. Irritates me to no end!!!
