A quote from the Link:https://www.opelgt.com/forums/gt/55778-gt-body-unit-production-numbers-5.html
The U“ stand for body dedicated for the US market. In oposite to the european GT, this bodys has to have a cutout in the quarter panel for the sidemrkers.
ALL GT-bodys inclusive 12 prototypes were made in France and „usually“ also painted end equipped at the French based factory B&L. The assembling of drivetrain and axels for ALL GT happend in the german factory in Bochum.
But there is one big exeption. Doe to high costumer demand, exact 24 043 blank bodys came as „body in white“ to the german faktory in Bochum. This bodys were painted and completely assembeled in Bochum. This happend in early 69 antil the end of 1970.
So you can say the were parallel two production lines. The body Nr for both lines are independetly count and in a different way marked.
What it makes a little bit complicated ist he fact that the mark for the „second“ line (i.e. in Bochum painted and completely assembled cars) differs between 69 and 70.
In 69 it is a „BB“ after/under the body Nr. and in 1970 it is a „20“ as a prefix before the body Nr.
1969 GT from "Bochum" production line
I read that post, but clearly missed the significance of the words. Huh, 24,000 GT's bypassed the B&L line to be built in Bochum.