All interchangeable. Early globe tubes by RCA were UX201A, Cunningham CX301A. Later prefix was dropped and tubes were numbered 01A. At that time they were changed to ST shape.
Norm
:Are the 01a and 201a (even 301a?) interchangable. I looked for cross references on the tube substitution page and can find only the 01a listed. The 201a doesn't come up for me.