I have analyzed enough 240 Series Volvos to conclude that there is a trend, in later years, to use Torx fasteners. There is an interesting write-up on Torx, on Wikipedia.
On my 1991 245, the tailgate attaches to the hinges with Torx T40 fasteners, but on a 1990 car that I analyzed, 12mm hex bolts were used instead.
It’s not as if any one year was the magic cut-over year to start using Torx. On a 1990 model that I analyzed, the side panels to the front center console were attached with Torx screws, whereas on an 1983 model, Philips screws were used instead.