Not much confusion...the right-most number:

x5 = instant B&W print and neg
x8 = instant color print
x9 = extended range instant color print

The left-most digits
5x = 4x5"
55x = 4x5" film packs
66x = 3-1/4 x 4-1/4
77x = 3-1/4 x 4-1/4 pro emulsion of 600 films