I've seen these referred to as "tone arm supports" as well as "tone arm support brackets." Either way, reproduction tone arm supports are being manufactured for Orthophonic Victrolas, since the original was made out of pot metal which can crumble in time. So far, no new production parts are made for Silvertone Truephonic phonographs, which by the way, were sold exclusively through Sears & Roebuck starting around 1926.
Chances are, the original part was made out of some type of pot metal, and a replacement culled from a junked machine would probably be the only option.
You may want to get in touch with the repair shops listed on this site, under the ACCESSORIES/RESTORATION section near the bottom of the page:
http://www.proaxis.com/~settlet/record/links.htmlIf you find a shop willing to help, you'll probably have to send the original part, for a match-up, or a good quality digital photo of the tone arm & the support bracket, along with detailed measurements, etc.
If possible, also post a few photos of your machine here, since Truephonic models don't surface very often.
BTW, Truephonic phonographs, as with Victor Orthophonic and Columbia Viva Tonal models, were specifically designed to play the (circ 1925) newly introduced electrically recorded records with greater fidelity than older style talking machines. Most are fitted with long exponential horns and tone arms, along with sound boxes equipped with aluminum diaphragms. To get the best possible sound quality--especially bass output--the sound box should be overhauled with fresh rubber gaskets, and adjusted for maximum compliance when tracking a record. In addition, any air leaks in the tone chamber, especially seam gaps in the horn, should be sealed to make the entire tone chamber as air tight as possible. You may be surprised by the before and after results.
Good luck!