It took a bit of hunting around online, but I was able to locate a lock retainer for the '63 door, in stock at Corvette Pacifica. It's the one on the left, next to the wrong one ('64 - '67) which I had temporarily installed on the new '63 door, in the first photo below. Second and third photos are the correct retainer installed on the '63 door.
Thanks for the help, guys.
Live well,
SJW