What is "System screws" and how it works? read more about it in article.

mere size is not the only factor determining the physicial compatibility of system boards however otherwise you  might  consider putting a smaller board into the box made for a larger one waiting to frustrate system boards are nearly the same size.the pattern of their mounting holes do not match.the variations often amount only to a fratction of an inch but they are enogh to prevent the easy substitution of one baord of an other.
IMB is amazingly frugal in the hardware used to hold its system boards inside their cases.as few as two screws hold them in place there are good electrical reasons for this miserliness with the hardware proper grouding being paramount among them, the balance of the mounting holes in the IBM's system boards are devoted to nylon fasteners which insulate the boards from the metal chassis while holding them in place mechanically the screws holds the IBM system boards in place,the nylon fasteners are designed to space the board vertically and fit special channels in the metal work of the case allowing the boards to slide into place.
IBM carried this same hardware frugality over to its,PS/2 line Although PS/2 planar boards wildly differ in shape and size between the various models they share the same parsimonious use of screws to hold them in place..