Is there a hole or indentation on the underside of the top strap(just above the barrel forcing cone) which lines up with the forward holddown screw of the sight?
Yes, it's the hole for the sight screw, it goes all the way through the frame. My 66-2 has the hole just above the forcing cone. My 66-4 has the hole just back from the forcing cone, where it's easier to see, plus two other factory drilled through and tapped holes for a scope.
Seems kind of weird for S&W to drill holes in a somewhat critical part of the frame, but apparently it's not a problem.
Here's a picture of the hole on a 66-4, it will be the same on the later versions too.
On the earlier versions (66 thru 66-3), there were no screw holes through the top strap.
I doubt the holes through the top strap on the -4 and later 66's are a problem but it does cause carbon to build up on the outside of the cylinder that I'm not fond of.