Does this mean the block is not original to the car?
No
Is it a 1965 c-code/ k-code?
Don't know - since all you have offered is the casting number from the block and K codes didn't use a different number than non-K codes.
Look for a VIN stamped on the block, open it up and identify the parts, Main caps, rods, pistons, cam, lifter.... of course some of the original parts that would help identify would be replaced or gone if its been rebuilt
You don't offer offer a picture of the head - guess you have not identified them as K code original ones. These may just be non-performance heads that some one had built with screw in studs and bigger valves. Chevy valves were popular for some builders. to squeeze a slightly larger valve into a small space