Yep, the 6906 had an alloy frame. This is one the numerous eccentricities in Smith's numbering system. With the stainless slide and alloy frame, it probably should have been called the 6903.
As said earlier.......5906 is all stainless........ I had one and traded it for a 4006 (.40 cal). They are not nearly as accurate as my Glocks, but were very nice IMO.
For what it's worth, I feel all of the 4 digit S&W semi's are excellent pistols. Had a 5904 that was flawless and currently own a 1006 10mm that I wouldn't trade for the world. Matter o' fact it is my home defense gun. I've always like the 6900 series and wouldn't mind picking up one of those inexpensive police trade ins I see now and again.