If you mean it does not have a "D" or "S" mintmark . It was made at the Philadelphia Mint. No U.S. one cent coin has ever had a "P" mintmark.
A letter D just below the date is the mint mark for Denver.It is called a mintmark, and it signifies where the coin was minted.
On average, 3 cents or so.
Average value is 3 to 10 cents.
If you mean a "D" under the date, it's not an error, it's the Denver mintmark were the coin was made.
A 1941 penny without a mintmark means it was minted in Philidelphia.
The Lincoln Penny was produced in both Philadelphia and Denver in 1956 (the latter have a "D" mintmark below the date).
There is no mintmark on the 1921 British Penny, they were all minted at the Royal Mint London.
On a US coin, an S mintmark means it was minted in the San Francisco Mint.If it has a D it was made in Denver.If it has no mint mark letter it was made in Philadelphia.
It depends on the mintmark and condition. A 1928 penny with no mintmark (thus minted in Philadelphia) in not that great of condition might only be worth $.10, however if it has a D (Denver) or S (San Fransisco) mintmark it could be worth a dollar or so in the same grade. If you have a perfect example it can range anywhere from $90-3,000 depending on the mintmark, but those are only for the absolute best coins, if you picked your coin out of circulation on average it would be worth $.10-$.20 with no mintmark and I'd estimate about $1-2 if it has an S or D mintmark.
A Lincoln cent without a mintmark was struck at the Philadelphia Mint. The Mint does not use "P" mintmarks on penny's.
A mintmark is a small identification mark used to tell which mint it was made in.