The densities of carbon steel and stainless steel are so close (within .003 pounds per cubic inch) that I doubt that that the scale that the guns were wieghed on could measure the difference!
I think that the grips could account for the difference.
My model 638 came with rubber grips, which are heavier that the older, standard, wood grips.
A bigger consideration for me would be:
"When was the gun made? and
"What mechanism does it have?"
The newer S&W's may have:
1) MIM triggers and hammers
2) The firing pin mounted in the frame, not on the hammer nose. That requires a heavier mainspring, and correspondingly heavier action.
3) A new internal locking system:
http://www.smith-wesson.com/misc/ilock.html
These 3 features are all negatives in my book.
The M637 is more likely to have some or all of the new features, and the M37 is not likely to have these features.
Unless the M637 is old enough that it doesn't have any of the new features, I would go with the M37.
-Mk.IV
