Pete... like everyone has said 50 years and older for your K-38.
One of the things about the term C&R that lends to the confusion about a firearm's eligibility is the routine use of the term C&R as a sigular description for something that is actually comprised of 2 provisions in the Federal Firearm Regulations... and I run into that often at gun shops while attempting use my C&R FFL for a purchase/transfer.
A curio firearm is one that has to possess some unique feature/characteristic or monetary value derived from a historical significance, in order to be listed on the BATF "C&R" list. By that definition the list published by the BATF is actually a Curio firearm list because firearms do not need to be 50 years old in order to be on the list. A prime example are the S&W commemoratives found on that list.
The curio list at times is not all inclusive, as firearms can be become a "curio" by submitting a request to the BATFE, and those grant such status are not shown on the list until the next update.
A relic firearm is what most of us are exposed to and purchase... 50 years or older and that's all it takes to be classified a relic... like me
And they do not need to be determined by being on any list at the Federal level.
I made that distinction because there are states that do not exactly follow the Federal definitions for Curio and Relic firearms.