Model 5X3 is a superheterodyne receiver. It originates from 1936, employs 5 tubes (6A7, 78, 75, 43 and 25Z5), has an IF of 460 kHz and covers two bands.
Electrically models 5X, 5X3 and 5X4 are identical, however the cabinet dimensions are different.

A schematic and further details are given in Rider's Perpetual Troubleshooters Manual (Volume 7).

A picture of model 5X3 can also be found in the book (or CD-ROM) "A collector's guide to Antique Radios" by Marty & Sue Bunis c.s. By comparing this picture with the picture of my own radio (see below), it became clear that my radio was missing its base. A USA owner of the same radio was so kind as to send me pictures of the base (thanks Doug !), so to enable me to reconstruct the base one of these days.

Left: my radio

Right: radio complete with base     (source: CD-ROM  Marty Bunis c.s.)