I pulled the engine and trans off the frame this evening and toted the frame out of the garage for a good look. It still gave me no clues on the age of the frame.
Thankfully,he very helpful folks at http://www.triumphexperience.com showed me that the car frame matches the chassis (unlike the engine and trans).
This confirms the frame matches the chassis, not the engine and trans. This leaves me with decision on the drivetrain, but that’s no rush.