George, In addition to Whitworth threads, other possibilities are BSF (British Standard Fine) or BSC (British Standard Cycle), both of which use a 26 thread per inch, 55ยบ angle for 1/4" diameter bolts. 26 tpi can look a lot like 1 mm (25.4 tpi), especially if you don't try matching them over more than a few threads. My 1975 (English) Condor uses a 5/16 BSF (22 tpi) bolt for the integral (fastback) binder, so their use in '72 is certainly possible. One oddity of BSC, is all threads 1/4" and larger, are 26 tpi. Your measurement of the diameter of the threaded portion makes me think it is metric, though, as 1/4" bolts of any thread form are likely to be over 6 mm in diameter.
Davis Jensen Lomita, CA, where it was in the 70's today as I enjoyed my 30 mile ride around and over the Palos Verdes hills.
P.S. Merry Christmas to all, especially to Aldo Ross for his great story.