Quoted:
For a 1964 Python you'll need the Colt Second Type Target grip.

This has the "half-moon" checkering border below the Gold medallions and the "thumb rest" only on the left side.
These are EXPENSIVE these days.
Fortunately most of the fakes look different from the originals and are usually easy to ID as fakes or "replicas".
Fakes almost always have brass screws and grip screw bushings.  Genuine Colt have steel screws and ONE steel bushing on the right hand grip.

Original Colt Second Type Target grip.
Note the semi-circular checkering border under the medallions.  Python medallions were always Gold, never Silver.  Made of American Walnut, usually with at least some better grain figure.
Steel screw and bushing.
"Thumb rest" actually a loading clearance cut only on the left grip on the Second Type.

http://img.photobucket.com/albums/v465/dfariswheel/SecondType.jpg

Quoted:
I buy from these guys. I put some rosewood finger groove grips on all of my wheel guns & they're both gorgeous as well as functional.

I have no connection other than as a customer, FWIW.

http://handguns.sdsnetworking.com/Python_=_Colt/352/c/1846