I have no idea how its backed, and to be honest, i cant see a reason to remove it, unless you were going to linen back it, which would sort of be pointless!
I am sure that in the mid eighties when this poster was in a frame at palace pictures no one gave a shit about what type of glue was was used and whether it would put of a poster collector 20 years into the future!
Its been a hard one to price, great provance, never folded but backed into thin cardboard, I have a goodfellas one sheet thats exactly the same that i got from the offices of Sky