How old is this S&W model 64?

I just picked up a S&W model 64 .38 Special. It looks a little old. As the barrel looks different compared to the more modern model 64's. Just wondering if anyone can tell me how old this puppy is...

Swmith & Wesson
Model 64
# on Cylinder Hinge - 46170
# on Base of Grip - D4919## (Don't want to give it all away)

Thanks!

I like things that go "BOOM"!!!