Craig, I suspect that the National 'line' ended sometime in the 1930's.
The 1941 Catalog in our .pdf section has no references to National, and I have a 1930's catalog (#58) which does not contain any 'National' items.(still trying to date this one more specifically, it appears to be mid to late 1930's).
I Suspect that The National line may have ceased sometime in the early to mid 30's, but that is an educated guess.
If the copyrights are ever lifted from the 'trade journals', we will likely know more specifically.