I briefly "Googled" KODAK GLASS PLATES P1500". It appears your plates are probably intended for scientific use with a lab instrument called a spectrograph. (I dunno what that is...) Here's a Flickr link to someone who tried exposing and developing Kodak-UK 4x5 plates that appear to be very similar to yours.
http://www.flickr.com/photos/richard314159/2354518775/