Here's pics of some of the differences [improvements?] that Aimco made. First pic shows the captive ball and coil spring that replaces the takedown lever hairpin spring. The extractor has lost its 'wings', the toggle knobs have no grooves on the outer diameter, and the toggle axle pin has a wide thin head. Oddly, the axle pin is not full-width; it is about ~.083" short...
Second pic shows the firing pin - it has only two flutes, 180º apart. The sear bar is not cut out; the frame has a removable pin to retain the sear bar.