No mintmark on a 1974 cent means it is from Philadelphia. All cents except 2017-P do not have a mintmark for coins from Philadelphia.

There have been cases where the mintmark was forgotten.

But not a case of a forgotten mintmark on a cent, but just means they are from Philadelphia. There are other coins that are missing mintmarks on other denominations:

But most of these are on the proof coins missing the 'S'. (1982 missing 'P' is the exception for dimes and this is on a business strike coin) Before 1980 the coins missing a mintmark are from Philadelphia. During the 1942-1945 nickels there were war time nickels with a large mintmark on the reverse:
In 1942 there were type one and two nickels:

They type one was regular composition. The type 2 were the silver
War Nickels. (note the large mint mark above the building?)
War time reverses:

There were also some counterfeit nickels during the war years that were counterfeits. Called henning Nickels:



But the counterfeits are worth more than the real nickels. (why?)
Proof coins before 1964 were from Philadelphia without a mintmark.

In 1968 forward the proof coins were showing a "S" mintmark. But from 1968-1974 there were also Business strike coins with the 'S' mint mark.


Looking at the fields shows you the proof VS the business strike coins.
CoopHome Missing Mintmarks. - henning - proofs