Barry Sunshine, a collector of California Fractional Gold coins and a student of the series, has this to say about the BG-101:
BG-101 is an octagonal quarter dollar issued by the Joseph Brothers (they are believed to be the first firm to make California Fractional Gold coins according to Nathan Joseph’s statement made in 1924). The authors of the Breen-Gillio book also believed that this example was one of the first and they listed it accordingly as a BG-101. BG-101 is a Low Rarity 5 with about 75 known to exist. There are a good number of Mint Stateexamples that exist, so finding a Mint State example should not be difficult with some patience. Normally, BG-101 comes well struck except for the 8 in the date, where it is very weakly struck.
Jack Totheroh Collection - Holabird-Kagin Americana 4//2013:100, $4,700