Standing Liberty Quarter Needs a 1930-S quarter added

In case it wasn't obvious, he's talking about this coin.

I think the mintage for the 1930-S is 1,556,000.

yes, you're right. I added it  
do you know if "S" mints (San Francisco) were Proof mintages, too (like newer issues)?

The rule with American coins that "S" means proof has only been the case since 1982.  Before then, the San Francisco Mint used to strike circulation coinage as well.

The San Francisco Mint produced circulation strike coinage of most denominations from 1908 through 1955, and also struck gold coins as far back as 1854.  Since 1955, circulation coins bearing the S mintmark have been limited to cents 1968-1974, nickels 1968-1970, and dollars 1979-1981.

To my knowledge, proofs minted before 1968 did not bear any particular mintmark, "S" or otherwise.

Your S coin is a proof if it is:
* a cent dated 1975-present
* a nickel dated 1971-present
* a dime dated 1968-present
* a quarter dated 1968-present
* a half dollar dated 1968-present
* a dollar dated 1971-1978 or 1999-present
Otherwise it is a circulation strike.

Here's some info on the SF Mint's history:
http://en.wikipedia.org/wiki/San_Francisco_Mint

...and on US proof coins in general:
http://en.wikipedia.org/wiki/Proof_coinage#United_States_proof_coins