Too many variables to be answered simply...but I'll try.
If we start in the early 1970s, Husqvarna in Huskvarna only made professional saws. The 140, 163 and 180 had the fuel tank & rear handle as a rubber-isolated separate cast metal assembly.
When Husqvarna decided to make the semi-pro 'Practica' 40 and 61 they followed suit, derived as they were, from the 140 and 162.
When the dipped their toe into the farm saw market in the '80s, they took the Partner 500 parts list and turned out the 50, again following the pro and semi-pro pattern. It was only when they followed up the 50 with the 40 that a saw that a Husqvarna saw with the the fuel tank integrated with the crankcase (in this case, fibreglass & aluminium reinforced polyamide) was allowed to leave the building.
AFAIK, all pro saws today have separate vibration isolated fuel tank and rear handle.
