1921-S Lincoln Cent Strike Doubling

[Verify-12]

Here's a 1921-S (San Francisco Mint) Lincoln Cent.  It's a nice early example of strike doubling (machine doubling), and a more extreme example than is normally found on most early Lincoln Cents.

[Figleaf]

I general, "double strikes" are caused by setting the press at a pressure that is too high. However, since only the date seems to be concerned, it would be logical if in this case the cause is a date punch that was used a bit too forcefully to complete the die.

Peter