Key the year as seen from the received date. The received date will typically be found in the "Date Received" column. Please only expand 2 digit years when reasonably certain of the century.
From 5 images you can ascertain which century it is. You are not pulling information from another image but looking for clues as to which century it is.
If someone doesn't feel comfortable keying a 4-digit year they don't have to. But I've done hundreds of these and I know which century it is.