RM-937 is the "International" variant with the Euro style radio.
You are correct, it should have 32GB as only the RM-940 ATT had 16GB.
Are there pics? The ATT model also has the 3 contact pins on the back for the PMA charging cover.
Like this:
With the pins it is ATT, Without it is any one of 3 variants - all of which have 32GB internal.
Can you get the seller to send a screen cap from Storage Sense?????
A screen cap from Extras & Info will show you the RM#.
http://www.gsmarena.com/nokia_lumia_1520-5760.php