Thanks for your rapid reply Dave. I noticed some of the nano pictures show a white frame, mine has a dark almost transparent frame that does not slide easily. 
However I was eventually able to slide it off, per your directions, thank you! Having difficulty putting it back together. 
Guess I’ll wait until I get it calibrated. Your guess as to the model version and installed firmware was also just as you predicted. Thanks again for your assistance.
Have a great day!
Charley