S&W 645/4506

What is the difference between the two? As far as I can tell, they are identicle. They are both all stainless, slide mounted safety/decock, 5" barrels and use the same magazines. Is this a similar case to the Desert eagle 3rd generation, where the model desiganation changed but not all of the guns did? (with the DE's, the Mk VII .50 AE is the same as the Mk XIX, but the Mk VII .357 & .44 did change when they went to Mk XIX). It would be especially helpful if someone owns both the 645 and .4506 and can relate any difference (Mark C., are you listening?)

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