SF helped alleviate the demand for a couple years and minted some bullion ASE's. Since they were the bullion version, no mint mark. The only way to distinguish them was by sealed green monster box. It would have the SF Mint band around the shipping container. Once that band was removed, there was no way to tell.
Not all coins produced by the SF Mint are coveted by collectors. Some of the most notable are the 1893S Morgan due to its scarcity and the 1880S & 1881S Morgans (just to name a few) for their strong strike and booming luster, but you will probably find just as many collectors seeking out the C, D & CC mintmarks, too. For many years, the SF Mint was best known for producing proof sets. Chris
Basically they just make them there when they have to. Adding a mint mark would be extra time and another expensive for the manufacturing process so they don't. They just want to pump those out as fast as possible and get them off to the wholesalers and be done with them.
There is a difference.......as Chris mentioned above, the collector eagles are burnished. What that means is that the blanks (planchets) go through a polishing process before being struck. The resulting product is supposed to exhibit a matte-finish with a soft frosty look to the coin, but I'll be hanged if I can sometimes tell the difference between the two products without the help of the mint mark.