I had a 66-5 (just sold it to a forum member), and it seems to me like the barrel was almost a "bull barrel" profile....
A "heavier" barrel, is how I have heard it described...
Here is a pic from the front, so more experienced forum members can weigh in on this:
I also think that the profile is different from earlier models....that part on top of the barrel, that basically elevates the front sight....is shaped different.
On earlier models, "that part" started right where the barrel mated with the frame...on the -5 model, you can see that "that part" does not go all the way to the frame, but starts midway between frame and muzzle....
More experienced chaps than me will hopefully chime in!