were the originals finned?
Not necessarily - my '66 Chevelle Malibu has non finned drums on all four wheels.
On my '69 RS the front drums are finned, and I have to doulbe check, but I am almost positive that the rear drums are not finned.
If I can find it, I have an AC Delco drum brake manual which includes intechangeability and part numbers. Any other numbers other than the minimum ID of 9.590" and the G-10-454 number?
In your second picture, you can see a "knock out" in the casting which is there to create the opening for adjusting the brakes. I know that on the front finned drums on my RS that there are two of these knock outs 180 degrees apart. One of them has been opened and has the removable rubber stopper in it so you can get to the star wheel on the brake adjuster. The second knock out is still in place. On your drums, is there a second knock out 180 degrees opposite of the one shown in the second pic?