whats the difference between 78-81 and 82-87 el camino power brake boosters? They have different part#'s Why??

I have a 78 elco and I plan on buying a new master cylinder but want to use a booster i got out of an 84 camino but I had noticed the booster for the 84's have different part #'s then the one for my 78..What the difference between the two boosters?