It’s a bit difficult to associate the term “rare” with a coin that had a mintage of more than 3 million at each of three mints. The proof mintage is more than 4 million, so any way you look at it the coin is not rare, even if it may be hard to find.
Is it possible to get a listing of the rarity scales used by some of the old-time researchers? I want to use them to determine the value of my coins.
There is no direct relationship between rarity scales and current market values, so there is no way to use them to determine the exact value of a coin. Some of the older references attempted to establish values based on the scale used, but they are outmoded by discoveries of hoards of coins that in many cases have altered the rarity factor. Many new varieties have been discovered since the rarity scales were established, so you would be better off to use the Coin Market published in Numismatic News to determine general values and then consult with some of the specialty groups for information on the numerous die varieties that aren’t listed in the regular price guides.
I have a 1918-SM quarter. Why the “SM” mintmark?
You are confusing two different, and separate, parts of the design. The “S” is the San Francisco mintmark. The “M” that appears on Standing Liberty quarters is designer Hermon A. MacNeil’s initial.
Why don’t they go back to large mintmarks like the ones on the war nickels so a collector doesn’t need a microscope to see where a coin came from?
A good thought, but one that is not likely to get much support from the Mint. Although they have been inconsistent over the years, varying mintmark sizes seemingly on a whim, the present policy is to tolerate the mintmark as a means of identifying or tracing a coin to its origin. It has been only under extreme pressure from collectors that they have retained the tiny letters in use. The monsters used on the 1942-1945 nickels were among the largest mintmarks in the history of world coinage.