Revisiting the basis of dating Mint Of The United States At San Francisco Type I ingots this week. From my research to date, I believe that the Mint Of The United States At San Francisco Type I ingots date back to the early 1930's. The screenshot below from the American Numismatic Society's Website represents the oldest date of record with accompanying picture that I have found.
According to the ANS Collection Curator, the donation was received on June 21, 1939. There is no "letter from Mint" on the back of the box as noted in the text above but there is a note stating that donor, David Bullowa, reported purchasing the ingot in 1935 for $5.
Earlier Mint Of The United States At San Francisco Type I ingot references exist in auction catalogs but without pictures or individual ingot numbers; catalog descriptions are limited to weight and fineness. The earliest of these that I have found to date is in the M.H. Bolender One Hundredth Auction Sale catalog dated Saturday, November 30, 1935. Note that this listing also includes reference to an earlier auction record for price comparison but I have not yet found that particular catalog.
M. H. Bolender catalogs contain a number of similar listings between 1935 and 1937, and then in 1938 they begin to include the individual ingot numbers. Throughout ALL of these listings, (all of M. H. Bolender and the ANS) fineness of 999.75 is consistent. In no other series of Type I ingots other than the large font, curved stem nines, (similar to the ANS photo) does 999.75 fineness exist until the early 1940's. This, along with individual ingot numbers (when available) that are within the correct numerical range form the basis of my position that the large font, curved stem 999.75 are the earliest Mint Of The United States At San Francisco Type I series produced.
A few photos from my collection follow to demonstrate the consistency and reliability of characteristics throughout this series.