Coin collectors and non-collectors alike often find coins with a missing letter or digit. It is not rare to find a coin with a missing letter or number, and it doesn’t add much value, if any, because they have not been designated as a variety.
A variety is when the die has been modified on accident or on purpose so that it is different than the initial or intended design. One example is the missing D on a 1922-D Lincoln Wheat Cent. Mint employees did all they could to extend the life of these dies to save them and eventually they filed off the D mint mark.
Since there’s several of these found, all the same, then it became a cataloged variety. We now know there’s more than one die pair but there’s enough of them just alike that these 1922 no D Lincoln Cents have became a rare variety. However, the same can’t be said for missing date digits or letters.
Some Lincoln Cents are commonly found with missing numbers in the date, for example, the 1943 Steel Cent is known for a missing or extremely weak 4 in the date, and the 1946 Lincoln Wheat Cent is known for a weak 46 in the date. Nonetheless none of these have caught on as a variety and add no extra value to the coin.
In most instances a missing digit or letter is a mint error or damage and the only way one will get variety status is if it was removed from the die itself.
Various reasons a date digit might be missing are: Filed (polished) off by a mint employee, date digit not punched strong enough, grease filled (Plugged) where the die digit’s cavity is compacted with grease or polishing paste, lamination peel off, die chip, die wear or by post mint mechanical means like a coin counting machines or at the hands of a person just messing with the coin.
In any event a coin with a missing date digit or a number is not rare and doesn’t add much value to the coin. I know that some have sold for extra value and some sellers are trying to sell them as rare at ebay. However I do advise caution when buying an mint error that is not being sold by an expert or is not certified.