Bob,
Do you do math problems when you are bored? That was great.

Something else to add in is the age of the replacement disc. How many clutches for mid '60's J trucks do you think your local NAPA has sold in the last 20 or even 30 years. It could have been sitting on his shelf since he refilled his inventory after selling one in 1969 for all you know. I think Bob's math is the answer though.