True, but it feels to me, and IMO, that since these were options that were generally made to a P15/10 model and not to the P13 "Professional Racing Bike" with tubular tires. We'd like to think that the LBS would have steered the original buyer to the P15 instead of letting him/her "downgrade" this P13 the way it has been. Just my 2 cents.

Part of the reason for me guessing this way has to do with the Shimano Crane GS (same as Schwinn Approved LeTour) RD. It was introduced in 1976. Maybe there are date codes to be found on the shifters, brake levers and calipers which could help pin down when they were added.