It’s hard to tell the exact reason for that screen. The valve base orientation can’t be seen from the pic, but this grounded metal plate is a screen between pins on the base, almost certainly to reduce RF coupling to later stages. This valve is the IF amplifier, so maybe the earlier stage has high levels of original RF frequency, and they wanted to suppress their transfer to later stages.
This really isn’t much of a problem at broadcast frequencies, but the receiver can receive signals up to 22 MHz, where inter-stage screening becomes important.