Ron Guth:
Proof 1828 Quarter Dollars are extremely rare. We know of six different Proofs of the Browning 4 variety, including one example in the National Numismatic Collection at the Smithsonian Institution. Steve Tompkins described a single Proof Browning 1 and Rea et al list a Proof-60 example of the Browning 2 variety (though it is questionable if that coin would stand up to today's standards and requirements for a true Proof). Thus, the maximum number of Proof 1828 Quarters stands at eight examples, though the true number of Proofs is more likely seven.
Virtually all of the Proof 1828 Quarters are PR64 or better. John Jay Pittman's "Very Choice Proof" (now in an NGC PR66 holder) appears to be the finest. Curiously, a couple of the coins have similar color and toning patterns, indicating that they may have been stored together for some time.