National Art Radiators

Might depend on how old they are.

https://archive.org/details/Sweets1938Sec2630NationalRadiatorCo/page/n13/mode/2up

This catalog appears to have been published in 1938. Interestingly, National Radiator Corporation looks like they were one of the first (if not the first) to standardize slenderized radiators, but the rest of the industry only adopted a large tube type standard on March 1, 1940 due in large part to WWII, and the slenderized rad standard was adopted later in 1943 with the Commerce Department's Simplified Practice Recommendations for Cast Iron Radiators:

https://babel.hathitrust.org/cgi/pt?id=uiug.30112109610573&view=1up&seq=5&q1=national%20radiator

I'm not sure exactly when NRC first started manufacturing their National Art line though. There is no mention of National Art Radiators in this 1930 catalog:

https://archive.org/details/NationalHeatingGuide/page/n7/mode/2up

or in this 1931 catalog: https://digital.hagley.org/Trade_Cats_1032013_045_91031?solr_nav%5Bid%5D=a42f9c81a40d43f65648&solr_nav%5Bpage%5D=0&solr_nav%5Boffset%5D=2#page/4/mode/2up

That means NRC started making them sometime between 1931-38. There is an EDR sizing difference between the Simplified Practice Recommendations table for slenderized rads adopted in 1943 and the sizes in the first 1938 catalog. If you want to be sure, your best best is to contact the Smithsonian:

https://americanhistory.si.edu/collections/search/object/SILNMAHTL_33015

Send an email to NMAHLibrary@si.edu. Reference the above collection. Ask them to send you a scanned copy of any NRC catalogs from 1931-1943 that include National Art Radiators. If the rads are definitely newer than 1943, let me know.