The number ususally means the length of wood it would hold. Glascock made a 16, 20, 25, 31 & 38 Box heater called a Charter. The #20 was a very popular model, I have about a dozen Glascock #20 Charter Box heaters.
Per by records - Martin Stove & Range Co. Florence ALA in my 1932 director, also listed in 1948 & 1952 editions. In the 1948 & 1952 listings you also have Martin Stamping & Stove Co. with same officers. These same officers are listed under King Stove & Range Co. Sheffield ALA for 1948 & 1952
I'd say two places might have catalogs you can copy related to this Box Stove.
1) The Henry Ford Museum
2) Clifford Boram of IN
The Ford Museum has lots of records that you can go through when you visit and you can also do some research on the internet.
** Clifford's address is:
421 N. Main Street
Monticello, IN 47960-1932
Send him a SASE and don't get in a hurry for a reply, he will send you information as he finds it.
Good Luck,
Nollie Neill, Jr.
NC