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This is a most likely a CVI plate as the two plates axis are marked with orange for the fast axis and black for the slow axis.
After determining that this is a zero order half wave plate, I tested with various visible and IR wavelength lasers through a Fresnel Rhomb prism in front of appropriate polarized beam splitter cubes.
It functioned best with my 405nm (near UV) laser. It also worked slightly with 450nm. So it seems to be designed for the beginning UV spectrum of 405nm and beyond into deeper UV such as 355nm YAG harmonics.
I have two of these for sale that act identical.
![CVI? 405nm + 355nm UV? 1/2 wave Ø1/2\](img-large/g/QDcAAeSw72hqj1JQ/s-l1600/CVI-405nm-355nm-UV-1-2-wave-1-2-zero-order-wave-pl.jpg "CVI? 405nm + 355nm UV? 1/2 wave Ø1/2\")
