So, now the detective work gets that much more interesting. The cabinet on my radio was falling apart when I bought it, and 2/3 of the bottom was missing, but we managed to bring it back just far enough to be sturdy enough to house its speaker and chassis and be completely solid.
The Collectors Guide to Antique Radios mentions nothing about a Mercury company, or where it might be headquartered. It does have a Rola model K speaker....25 ohms!
Now this kinda stuff sure can make a hobby fun