It is a 638 S&W and,it does have a shroud on it and your 637 is an EXPOSED hammer for s/a[single action,cocking hammer back] or d/a[double action for just pulling trigger On the 638 there shows the top of cocking hammer where you can still cock it like your 637 and they are really both the same the only difference in the 638 is that it is shrouded. rz625-8