Seeing anyone knowledgeable of Lufthansa history!

I’ve tried everywhere I can find online, in books, and anywhere else, and I can’t find the definitive answer to this question:

Which specific Lufthansa 737-130 flew the first revenue service in 1968? I’m looking for the registration of the exact aircraft. There were two of them in Germany at the time (D-ABEB and EC, I believe), but I can’t find out which one actually flew the first service.

Need this information for a forthcoming book.

Tks!

JH