If I were in your position, I'd also opt for the generic one. The Silvertone branded sound box you posted is definitely for a cabinet model, probably one from the 1923-1925 period. It's more stylized and simpler in construction than the earlier ones used, and is totally inadequate for later-production 78s, which had greatly extended range compared to the acoustic recordings, for which the sound box was originally designed.
The generic sound box looks to be from about the late 1930s, and should do okay.
By the way, since Sears didn't own it's own manufacturing facilities for any of the products it sold, technically, all components used in Silvertone phonographs were "generic." They were manufactured by third party suppliers for Sears. (The cabinets were too.) Some phonographs, such as the Sears "Tru-Phonic" portables, were sold under other brand names.
Here, by the way, is one of the deluxe Sears portables, from around 1932 --
https://www.youtube.com/watch?v=s4uU9sy8aqk(Of course, it's playing some crappy record, with a worn needle from the sound of it. That, or just as likely, the sound box could stand a complete overhaul.)
Compared to others on the market, the Sears Deluxe models (some branded Tru-Phonic) were right up there. I don't know if you've seen it, but a few years ago Carsten Fischer, a well known Bay area phonograph enthusiast and restorer, created this page devoted to the best portables of the world --
http://myvintagetv.com/updatepages1/cha ... tables.htm Clicking on any of the images will take you to the page profiling the phonograph illustrated.
JDS