I had been wondering the same thing after I took delivery on my '76 late last summer. In the process of disassembling all the body fittings prior to a re-paint, I discovered that there is a removable steel plate on each frame extension that adapts it for the later-style (i.e. larger) black underriders. Once you remove this by undoing the two bolts used to adjust fore and aft positioning of the hood (bonnet), the earlier versions will slip right on. Later bumpers ('75-'78) should have the appropriate holes to attach the earlier underriders (mine did). Note: The lower attachment point (under the frame extension) did not match up on mine, although the upper two mounts seem to be sufficient to hold the pieces in place.
FYI: I bought a set of early style black underriders (replicas made of fiberglass) from a company in the UK called James Paddock. The set cost me about $80, including shipping to Honolulu. Cheap upgrade that looks great -- highly recommended.