The second most rare date of all U.S. Mint San Francisco round hallmarks is the 1960. In my research over the past six years I have only found four examples. This class ingot was produced in 1956, 1958, 1959 and 1960.
The most common date is 1956 which overlaps numerically with Type II oval hallmark ingots and makes the transition from Mint Of The United States At San Francisco Type II oval hallmarks to U.S. Mint San Francisco round dated hallmarks. I have never seen a 1957 round dated hallmark or found any reference to one. I have a photo of one 1958 that was in the Paul Franklin collection; that is the lone example I have found from that year. 1959 is second behind 1956 in commonality and then the four I mentioned above in 1960.
Recently able to add the round dated 1960 U.S. Mint San Francisco ingot below to my collection. At 32.41 ozs, it is among the heavier of these generally referred to as 25 oz class examples.