[Arran]

I wonder if the Mission Bell built Silvertone sets were designs exclusive to Mission Bell or whether the chassis were built according to Sears designs?
On thing I noticed is that the chassis designs and layouts of Mission Bell sets are very similar to Packard Bell radios from the same era? Packard Bell used dials and control layouts that are very much like Mission Bell sets, generic knobs, and Quam-Nichols speakers, though it looks like some used Lansing speakers too.
Packard Bell was the successor company to Jackson Bell, organized by the same Herb Bell. A number of California built sets were manufactured in the Gilfillan plant in Los Angeles because they had an RCA license, those that were not made by Remler that is. If Packard Bell used the same parts suppliers or design then it would be much easier finding a junker Packard Bell chassis then a junker Mission Bell chassis.
With regard to coil forms, no a toilet paper roll will not work as a coil form, I think I tried it once when I was younger and the turns will just crush or deform the tube once you put any real tension on it. One think that may work is a cardboard tube from a roll of fabric or wrapping paper, they are much more rigid, perhaps the tubing from a roll of Seran Wrap or aluminum foil, if the diameter is right. Plastic pipe or tubing may be another option.
Regards
Arran