i have probably seen more 3195/97 wheels with missing side rods than any other cast iron Flyer type. two problems plague these late C/I production locomotives. on many of the wheel hubs i notice the start of metal fatigue due to impurities which was even more evident in the next generation of production diecast locos. the side rods for these engines were held in place with a rivet which counted on a slight void behind the surface hole to expand and hold. many siderods were probably lost and never replaced from this rivet working its way loose, but i have also seen some home-brew examples of people tapping holes and using screws to hold the rods on, some quite successfully.
the other problem with the siderods was the eventual production change from stamped metal to diecast siderods. most of these simply deteriorated with time and sometimes you can actually find a rivet still set in the hole with the siderod(s) missing.