[Arran]

 I have a Canadian Philco model 42 which is similar to a 41-280, and one of the IF cans does have a threaded rod sticking out of the top for a permeable slug, but it also has at least one trimmer in the same can. So if the schematic shows this on the 1st IF can then it is probably correct for the 41-280-290 as well. If I remember correctly these sets have three If cans and two IF amplifier tubes as well, I was told that this was necessary with the set that used the XXD mixer-oscillator triodes (or sets using the twin triode XXD/14AF7) because the detector side offered less gain then either an octode or pentagrid convertor in the front end. It was also a way of delivering more gain and selectivity, with fewer parts, and a cheaper price then having a proper tuned RF amplifier stage.
Regards
Arran