Wahl Hard Rubber Gregg Fountain Pen section Removal

[wingwiper]

looking for a little insight as to am i missing something trying to remove the section so i can install a new sac? i've done the usual heat, soak, etc. and no dice? is it screwed in, is the section in the barrel rubber, celluloid or metal (that has rusted?). it is a cool pen i have had forever. removed the original nib many years ago and shoved a waterman #2 music nib in that gave me joy for many years. would like to put the original nib back in, but cannot remember which one it was?

any help appreciated