Philadelphia Mint Question

Can you post up a few pictures of the coins and MM's?

The earliest P mint marks were small, and difficult to distinguish from D. They were also punched by hand, leading to more sloppiness than we see now. During the 1980s, the marks were made larger and began being engraved, thus becoming more legible.

I would break the OP's question down into three parts:
- Why did Philadelphia mint suddenly start using mintmarks?
- Why were they so tiny and hard to read at first?
- Why did they gradually get larger?

I think the answer to the second sub-question is that they didn't want the mintmark to be too much of a distraction from the artist's original intent for the design. Don't forget, mintmarks weren't originally put there for collectors to collect. They were put there as part of the mint's quality control purposes. For these quality control purposes, mintmarks don't need to be legible once the coins become circulated, so the fact that you and I can't tell the difference between a tiny P and a tiny D once the coin hits below EF condition, is irrelevant.

In the 1970s, the mint was still coming out from the 1960s attitude that "coin collectors are hoarders, annoying pests who should not be encouraged". They gradually came to realise that coin collectors are actually good for their business, because we "hoard" coins that they then need to make replacements for, thus increasing their profits (so long as every coin remains profitable for them to make). And so they made mintmarks more prominent and more legible, for the benefit of coin collectors - which answers at least in part the third sub-question.

In the early years of putting the P mintmark on the coins (1979 to 89) the small P's were hand punched into the individual dies by hand (as were the D's) As they transitioned over to having the mintmarks on the master hub the mintmarks were made larger and clearer.