I don't know why it happens but the 4 on the 1943 cents was always a very common spot for die filling to occur. It doesn't happen on the other years in the 40's that often, just the 43. There are several coins that seem to have "favorite spots for filling in. The DE quarter, the last E in STATE, the CT quarter letters in Charter Oak, the NH quarter it was the mintmark, the Bicentennial half dollar the last E in INDEPENDENCE.