Back to the original question - or set of questions :
IIRC the original belt fed Heckler Koch machinegun designs used a disintigrating belt (for the 5.56X45mm designs) which was different from the Fabrique Nationale (FN) design which was adopted for the SAW/M249 and similar US issued machineguns. As I recall these belt links are not interchangeable between HK and FN designed machineguns.
I believe that there were HK modified designs - or a change to the actual product (referring here to the machineguns) - which allowed use of the FN type belts. I would highly encourage anybody looking to purchase an HK belt-fed machinegun to reasearch this topic.
As I mentioned with the M1917/M1919 design there were differences in belt types and feeding mechanisms. I was hoping somebody could follow up with more informaiton about the HK designs on which my memory is scketchy.