I went to the Independent Label Market in London yesterday and bought a few things from the 4AD stand. They slipped in a mysterious white label 12" and said I should be very careful with it and winked at me.
Got home, threw it on the turntable and it turns out it's the Dig for Fire 12" test pressing.
I just checked my archives. I recently saw the DFF TP for sale at 30 euros, the Gigantic TP at 28 euros, the Doolittle TP at 50 euros and the PoS WL at 15.
Distinguishing a test pressing from a white label might sometimes be difficult.
However, in the case of the Pixies' original 4AD vinyls, it is very simple as the test pressings have printed labels with the Mayking Records name and logo and the "test pressing" mention. On side A, the 4AD catalogue number (handwritten) and the date of the test pressing are added.
The white labels just have... white labels, with ou without handwritten information on the release.