The Hetzer was a 'new' vehicle built from the basic hull design of the 38t. The first Marder 3's on the other hand were simply converted from existing 38t stocks by removing the turret and superstructure plates, and adding 7 armoured plates for the new superstructure parts, the Russian 76.2mm gun on a cruciform support and adding a gun shield.
The next development was a redesigned mounting platform and side shields on the same basic hull layout - Marder 3 Ausf G/H.
Subsequent Ausf M models were built as new vehicles with the engine mounted in the centre of the hull, fighting compartment relocated to the rear, and german Pak 40 fitted. Chassis remain unchanged.
Production of the Marder's was halted in May 1944 when production was converted to the more robust and effective Hetzer design.
Like all German vehicles, improvements were made during production of Hetzer's as the crew's provided feedback following combat use became known. Right from the start the Hetzer had the slightly larger road wheels to carry the extra weight of the full superstructure and gun, and they were specificly designed for that vehicle. They always had the 32 bolts. Late model Hetzers did not have bolts in the road wheels at all - they had rivets instead, but it is possible that some were rivetted every second place, so the 16 bolt myth may be a result of that, although I have no references that confirm this ever happened. On the final production series they went one further and were simply welded.
Amongst the many variations during production run, the "Early' vehicles had the basic 12 hole Idler wheel, late production had the 6 hole Idler and the experimental "Starr" had a 4 hole idler.
Hope this helps