Is a Morgan silver dollar without a mint mark unusual?

Answer 1

Not at all. That means your coin was minted in Philadelphia. There's a list of values available at many sites; for ex. http://www.numismedia.com/fmv/prices/mordlr/pricesgd.shtml

Until 1979-80, Philadelphia coins did not carry a mint mark. The only exception was during WW2 when nickels carried the "P" mark due to a composition change to save metal for the war effort.

In 1979 the Mint decided that Philadelphia coins should also all have mint marks. The "P" mark was added to dollar coins in 1979 and to all others except the cent in 1980. It was decided to leave the P mint mark off the cent because small numbers of cents are sometimes struck at San Francisco and/or West Point when demand warrants. The Mint did not want to create artificial rarities versus the enormous quantities struck at Philadelphia and Denver