Are the fast axes on Thorlabs quarter-waveplates mislabeled? Some members of my lab are performing a polarization-sensitive experiment where they need to use a quarter-waveplate (QWP) with the fast axis in a specific direction.  In the process of carefully checking all the polarization optics, they discovered that for QWPs from Special Optics and Thorlabs, different optical axes are labeled as the fast axis.  They have done a number of tests (1) and came to the conclusion that the ones from Thorlabs are mislabeled, and what is labeled as the fast-axis is actually the slow axis.  For most experiments this wouldn't matter or be easily noticeable, but in our case (and many others) it can cause the sign of the measurement to be incorrect.
After a lot of back-and-forth, Thorlabs now agrees that their QWPs are incorrectly labeled, but would like to confirm this with another research group or industrial lab.  Does anyone have a QWP from either of these companies and the ability to determine if the fast axis is correctly labeled?  If so, could you please answer here and let us know the model number of the QWP and how you determined the actual fast axis?  Thanks!
1) Including looking at the magnetoluminescence from CdTe and the phase shift induced from total internal reflection from a glass prism. 
Edit: Just to clarify, this is not any sort of interesting conceptual question about waveplates, but rather a practical matter that may be relevant to a large number of scientists performing polarization-sensitive optics experiments. 
Edit2: We would also expect the thicknesses of the two components to be slightly different.  We took some high-magnification images today and it does look like the marked edge component is slightly thicker, which also suggests that they are mislabeled.  


 A: We have obtained a Fresnel rhomb (FR) from Thorlabs to test their QWP in the manner suggested above (by ARM) and came to the conclusion that the wave plates are indeed labeled incorrectly. Our procedure is as follows: when the true slow axis of the QWP is collinear with the P-component of the TIR in the rhomb (labeled by Thorlabs as the rhomb's "fast" axis), the polarization of the light passing through the QWP+FR setup should remain unchanged.  
A: I also recently found that an entire order of Thorlabs quarter wave plates (WPQ05M-780) had a mislabeled fast axis. I initially suspected this was the case when I saw that they gave an opposite rotation to other wave plates I had, both older ones from Thorlabs and ones from another manufacturer (Meadowlark optics). When I looked closer, the marking on the optic itself (which has a little flat edge indicating the fast axis) was perpendicular to the line indicating the fast axis on the mount.
It's a very strange problem. I can only imagine that some miscommunication led them to think that the axis marked on the optic was actually the slow axis. But it is bizarre that such a trivial problem would last for four years and multiple people (including us) contacting them directly.
