Welcome Yoyo. Check that code a bit closer... the "L" should be a numeric, I suspect a '1". Alembic serial numbers about this time are arranged such the first two digits identify the year of completion, the letter code identifies the model, and the four-digit number identifies the instruments place in sequence. USA, of course is the country of origin, where all Alembics are made.
For instance, 91 U 6583 USA would be a Europa bass, made in 1991, and approximately the 6,583rd instrument made since 1972.
Sounds like someone thought very well of you. I'm not familiar with current pricing for secondhand Alembics in Germany, but we have a couple Club Members here who are closer that may be able to help out. Good luck with your sale.