Vintage Unusual Stereoscope Card Set 50

Description

Vintage Unusual Stereoscope Cards (50 total)
This is an unusual set of stereoscope cards. They seem to be made for some sort of testing, maybe for an eye test or for early eye training, perhaps. I wish I knew more about them but I don't. They are organized in sets that get progressively more complex. I do not know if the set is complete. I am guessing there were more cards as the sets are labelled with letters and numbers and there is not a set B.
If you happen to know what these are, please feel free to let me know even if you aren't interested in purchasing them.