It may be be covered with lath and plaster as well which would open up another can of worms.
Insulating window weight pockets.
Remove exterior flat casing to reveal weight pocket.
I did this on several windows.
Then cut strips of 2 foam board to fit into the wieght pocket where the weights used to be.
That will address any issues with the weight pockets.
I would not try to fill in the weight pockets with any insulation.
Use foil tape to seal the joint between the window jamb and foam insulation.
For window weight pockets i just blow out as much dust as possible then spray the area with water then canned foam the cavity.
Use a hammer and pry bar to carefully remove the wood trim from around the inside of the window exposing the sash weight pockets.
Check the fit of the brick molding.
Insulating window weight pockets i am not 100 sure that just removing the casing will reveal the cavity.
We use a router to grove the sides of the sash so that the sash can move along an interlocking metal track.
Remove sash weights for upper sash the ones closest to exterior wall and replace pulleys with the pullman device that fits into the pulley hole.
See below for a shopping list and tools subscribe to th.
Stack strips to completely fill the void.
I restore these windows for a living.
Cut strips of foil faced rigid foam insulation to fit into the weight pockets.