Jim's answer is correct, except that a pre war Model 1905, 4th change, does have a safety hammer block, of the 2nd hammer block style. Post war S series serial numbers have the third style hammer block, designed during WW2 to replace the 2nd style, after a US Navy sailor dropped his loaded WW2 Victory Model on the deck of a ship and it discharged, fatally wounding him, because he had not cleaned the gun and it was full of cosomoline grease that jammed the hammer block from working correctly. Redesigned Victory Models with the third ( new ) hammer block were stamped with an "S" to signify the revision. Ed.