Thanks all for your replies and thoughts.
In my particular 335, the full thickness center block seems to extend from the end pin to just past the bridge posts, which, yes, means the bridge pup screws are only attached to the top. I know! Crazy!
I have done my best to get a photo with my iPhone inserted into the upper F-hole and a flashlight. These are the best I could get. The end pin is to the right, neck to the left. You can make out the end of the bridge pup floating in the open cavity, and if you look close you can see a bit of light at the far side of the cavity coming in from the bottom F-hole.
When I’ve had the pups out before, I did notice there was a lot of empty space around the bridge pup. The neck pup, though, is nestled into a routed cavity in a solid block of wood. Makes me think there is a second short block just at the neck. I.e., two blocks, with a space between them under the bridge pup.
Finally, as you can see, there seems to be two plates, upper and lower, that the centre block is sandwiched between, and these plates appear to run the full length of the body cavity, endpin to neck.