Hi David,
I am not 100% certain, however, here is what I think.
The first date shown as übernahmetag (translated Date of transfer or Date of delivery) may mean the day the factory delivered the car to the dealership.
The second date 14 Sept 1968 may very well be the date the dealer sold the car to the first owner.
I think the factory does not release the car unless paid for in advance by the dealer. Your date on the car's nameplate must be close to the first date i.e 18 Jan 1968.
Cheers,
Rolf-Dieter