Ron Guth:
In 1945, the San Francisco Mint produced Dimes with either normal-sized or small-sized mintmarks. The smaller of the two mintmarks is so tiny that it became known as the Micro-S variety. The normal-sized mintmark appears to be the more common of the two varieties, although neither one is scarce. Both are readily available in Mint State, with or without Full Bands, though the 1945-S Micro S is a bit tougher to find with Full Bands. According to the PCGS Population Report (as of April 2012), the following are the finest certified examples known to exist at present:
1945-S: 1 in PCGS MS68
1945-S Full Bands: 3 in PCGS MS68FB
1945-S Micro S: 1 in PCGS MS68
1945-S Micro S Full Bands: 2 in PCGS MS68FB