Roger:
Good find. Reverse shows the Pyramid of the sun at Teotihuacan. In the background are the mountains of Ixtaccihuatl and Popocatepetl. Your variety was issued intermittedly between 1943 and 1955. Coincidentally, another 20C coin of about the same design was also minted in Mex. City in 1943. That one was made of .72 silver and 'said so' on the coin just under the eagle.
Don.....