Here's some discussion on a forum about key codes for pre-68 GM keys. There was a guy with a book and enough experience to to decode someone's key. If you strike out finding a locksmith with this old info, maybe you could reach out to him?
https://chevytalk.org/fusionbb/showt...hp?tid/141329/
This web site has some codes for pre-68 locks. There is also some useful drawings. Unfortunately, I didn't find anything about where the codes were stamped on the cylinders:
Pages from Lock Manuals
Here's a link to a useful description of post-67 lock cylinders. There's a page showing where codes are stamped on the cylinders and some hints at how to translate codes into keys. Unfortunately this does not cover pre-67 but may give you some insight and some hope. My guess is you'll find the codes in the same place on yours:
Ekl - GM Lock Presentation
Here's another useful site that shows the key groove profiles for the letter codes of the post-67 blanks:
all-about-gm-general-motors-lettered-square-and-round-key-blanks/