The wrinkle thanks were in the early K0 run...including the early sandcasts. At
some point in the K0 run they switched to a different manufacturing technique
that did away with the wrinkles.
The wrinkles can be seen by looking straight down into the filler hole and run
front to back where the metal bends over the frame. There are other wrinkles
further back on later tanks, but these don't count as a true wrinkle tank.
Because of there limited numbers and their "correctness" for early restorations,
they have become quite pricey.
Jim