Here you get a pretty good look at the elliptical 6" x 4" permanent dynamic speaker inside the SX-3. It's capable of producing reasonably good sound; and it also happens to be the same size as the speaker in the big early Sonys, including the TR-72, TR-74, TR-741.
The battery box accommodates up to six D-size (UM-1) batteries, which can be handy when you want to power a 9-volt radio. To deliver 6 volts to the 6C-3, the adjustable partition in the nearer battery holder slides to limit the holder's complement to four batteries.
You can see pretty much all there is to the back cover. The text in the center covers safety precautions. The 6V and 9V battery layouts are also kindly noted. Hopefully that prevented at least some people from frying good 6V radios. The coolest thing about the back cover is something I failed to photograph. I find the sweeping motion of the black metal latch release mechanism a lot of fun to operate.
And one last gratuitous shot of the speaker's face, along with its companion items.