its a question of "off-set", the New model cylinder, the case rims are "exposed" ,not recessed, into the rear of the chamber, like the 'old models', which also , have a firing pin groove as well..., thus the stud ( ratchets) in the rear will be deeper or vice verse, also in the front, the barrel stem may be 'set back, thus this calls for a LONGER 'stud' ( gas seal) up fronts to maintain the barrel/cylinder "gap",,,,,,then of course there is the actual "fitting" ( timing) of the hand ( pawl) to the racthets to make the cylinder "turn" when you cock back the hammer........ALL has to be in its "proper" place and sequence ( timing) lock up and bore alignment...............not rocket science, but all still critical for safe firearms operation..........won't know till one "tries" but should have all the proper measurements BEFORE you go out and pay cash for just any old cylinder, ( pig in a poke......) :roll: