Sadly, the bus line 69 doesn't run anymore, but we were lucky enough to get the full experience last October.
This bus is a 1960s vintage with the entrance in the back. A BVG employee would stand at the back of the bus and sell tickets, leaving the driver free to concentrate on the road. This particular bus was sponsored by Bodenheim, apparently it was Berlin's biggest carpet seller at the time. You'll find more signs at Signs, Signs.