Why large/small dates for 1970-S cents?

The 1960 Large Date was a deliberate change because the Denver Mint allegedly experienced die chipping in the Small Date. If you picture in your mind the recessed date in a steel die, there is a thin, raised ridge of steel that forms the recessed inside of the 9 and the 6 on the struck coin. Those ridges apparently were too thin to hold up, so they made a new Master Die with a larger date, enlarging the 0 as well.

(The Philadelphia Mint only made a few million of the Small Date cents early in 1960, which was standard operating procedure for every denomination in every Mint in every year at the time so they had some on hand, before it switched its Bronze coin production to foreign coin orders. By the time it resumed making Lincoln cents, the date change had already been made. See my COINage article on the subject.)

The difference in the 1970 coinage is much more subtle, and I do not think that there was any deliberate attempt to change the "size." For whatever reason two different Master Dies were made, and there were slight differences in the execution, most easily seen in the relative heights of the 7 and the 0. Go ahead and sign your name twice. I guarantee that there will be slight differences in what otherwise is the same thing.

Had the 1960 change never happened I am sure that very few people would have cared about the slight differences in the 1970-S cents. However, Dave Bowers and his company at the time decided to promote them heavily, and I believe lobbied to get them in the 1971 Redbook, which back then came out around July 1 every year. Once in the Redbook they were immortalized.

I cannot answer the OP's question as to why two Master Dies were made, or why the second one was only used in SF. I can speculate and say that a new Master Hub reduction had been introduced in 1969 (compare the 1968 and the 1969 versions, which are enormously different), and it was possible that somebody thought that the first version of the 1970 change to the 1969 version was a bit lacking somehow. The Engraver could have quickly made a new Master Die with an unintentional but frankly insignificant slight change in position, and so as not to waste them all of the dies already made which were descended from the first Master Die they were mint marked and shipped off to San Francisco for use, never to be heard from again. As I say this is speculation on my part.

TD