How did they get this wood up in the attic for the floor?

The house was built in 1950. At an unknown time, some type of fiber board was put down in the attic as flooring. In the 70's and possibly all the way back to when the house was first built, the former owners may have lived here for all of that time. Some young kids actually had their bedroom up there back in the 70's. Very odd, because the side vents are open to the outside air and it only had a floor furnace. Anyway, over time the junk board has cracked and turned into holes in some areas, while other pieces have stayed fairly whole and stable to walk on. I would like to rip all of that up and put down some new 4' x 8' plywood. The problem is… How the heck did they get it up there? There used to be some stairs in the living room corner that went up into the attic. Not sure how big they were, but the FHA required them to remove them before the loan could be approved. Instead, some fold-down stairs were put in with the typical pull-down attic door. That opening is maybe 3' long at best and about 2' or so wide. How the heck could I get this accomplished? Even at half-sized pieces, that would be 2' by 4' and would technically fit, but do they even sell such a thing as a common size? The floor joists are about 12 inches apart.
Added (1). Note that the size of the stairs apparently wasn't very big, either. There is a slightly visible mark on the ceiling where they used to be, which has now been covered with drywall. The stairs couldn't have been over about 3' wide at most and not very big. I doubt that 4' x 8' wood could have been brought up those, either.

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