All S&W handejectors, even back to the 1896 .32, have a method of keeping the hammer nose off of the live primer- most commonly done by the little "lump" on the rebound inside the action.
However, sometime before and during WW2, IIRC, they began installing "hammer blocks" in actions, which is a second line of defense.
If memory serves me, also certain completely enclosed hammer models such as the 640, 642, etc., do not have a hammer block simply because there is almost no likelihood of the hammer being "stuck," simply because it is fully enclosed?
I think thats right, but I don't feel like taking my 340 apart again right now.