It uses electromagnetic deflection. Most 9 to 12 inch prewar sets did. Electrostatic deflection was used in the 5 inch models.
It did have a back originally.
Here is a page about the restoration of the HM-225 in the Museum's collection:
http://www.earlytelevision.org/ge_hm...storation.html
There are links to pictures of the chassis and a list of tubes, etc.
We haven't posted a schematic on the site, but here is one of the RCA TRK-12, which is similar:
http://www.one-electron.com/doc_archives/TRK12.pdf