Thanks!
So I've been thinking about what the DL stands for, and going through the past catalogues I've come up with a theory:
I know the DL option came with Tantalus as well, and I think the DL was only offered in 88 and 89.
My best guess is short for "Deluxe". The 88 Tantalus DL was the option with better parts and Tange Prestige frame. In 89 the Tange Prestige frame was also an upgrade option for the Thunderbolt/Tantalus,
so maybe all the ones with Tange Prestige were labeled "DL" and the ones with Tange 4130 were the regular Thunderbolts...what do you think??