The 1903-S Micro S Silver Dollar is a variety (VAM-2) with an undersized mintmark, similar in size to the mintmark on a Quarter Dollar. Most likely, the engraver picked up the wrong punch, stamped the mintmark into the die, and missed the error.
The 1903-S Micro S Silver Dollar is popular with collectors and it can be found with relative ease. PCGS reports (as of September 2011) a population in excess of 350 pieces, but PCGS has not certified even a single Mint State example since 1986. The finest examples collectors can hope to find are a pair of PCGS AU-58s.