I've been collecting the state quarters so that I can have collections later on for grandkids, etc. Right at first it was apparent that it would be good to have both the Denver & Philadelphia coins for each, and have found books that allow slots for both.

Anyhoo, in my efforts I also came across the 1976 Bi-Centennial quarters, and thought it would be nice to add those, along with the golden dollar, etc.

In all of the time I've been doing this, I've come across only Denver quarters, but also have several with no mint stamp at all.

Did the Philadelphia mint NOT do these quarters? Or do you suppose that the quarters with no mark are from Philadelphia? Thanks!

According to this site:

"... the Bicentennial coins actually enjoyed a two-year run, replacing the regular versions of the quarter, half dollar and dollar in 1975 as well as ‘76. For all intents and purposes, there are no 1975 examples of these three coins, since no pieces carrying that year’s date were struck in the year’s early months. The Philadelphia and Denver mints cranked out combined totals of more than 1.6 billion Bicentennial quarters, 510 million half dollars and 210 million dollars for use in everyday commerce. The San Francisco Mint made more than 7 million proofs of each coin in the regular silverless composition and 15 million silver-clad specimens – 11 million business strikes and an additional 4 million proofs – of each denomination."

The blue emphasis above is mine. I left in the remainder of the paragraph as a matter of interest. More on the history of these coins is at the site.

The massive production requirement for bicentennial quarters was among the mint's major hurdles. Surely, both facilities were needed to produce the coins!