3 screw - Has 3 distinct screws in the cylinder frame, a slightly different grip profile and no transfer bar system - also called an old model (OM)
flattop - has a flat topstrap with no protective ears for the rear sight. Up until the 50th Ann BH was released all but a special NM were old models or 3 screws
While these can be collectable, it's really neccessary to have the box/paperwork to really complete it as a collectable and have the gun in pretty good condition. Some are more valuable than others because of production numbers. OM's that have been converted to the new transfer bar system are less collectable than those not converted. Any non-converted OM ent to RUger for anything, WILL be converted. Some folks ask too much and other pay too much just becuase they think them to be "rare". All depends on the model.