I have some 90-100% original bikes that may help to partially pinpoint the year:
1966 Carlton Flyer - no toestrap rub or tooth wear - has a headset which lacks the <C> on the upper and lower races, but has the dashes on the locknut :
http://www.ece.ubc.ca/
1970 Raleigh International - not even tires were changed - has the <C> on the headset cup.
1974 Schwinn Paramount - i am 2nd owner/no changes - has the <C>. 1974 A-series Raleigh Pro - has the <C>, only pedals/seat changed. 1974 International - has the <C>, only 1 tire/rim changed.
So in my opinion the <C> was added sometime between 1966 and 1970.
- Don Gillies
San Diego, CA