It started around the late 1940s with the Model 1 Compact ( not C1). Here are the models that followed (not including Revelation or Standared models as I'm not sure when they stopped making them)
C-2,
C-4 (introduction of ABC power nozzle and fully electric hose),
C-5 (introduction of swivel caster),
C-6 (now has double casters and was the last model to have the ABC power nozzle),
C-7 (Has Turbenette turbo nozzle)
C-8 (introduction of the Eureka made power nozzle with cord wrapped around hose)
C-9 (re-introduction of electric hose)
C-80
C-80-2 (last of the Eureka made nozzle and somewhere in here was the introduction of the Compact/Tristar made power nozzle)
Tri-Star CXL (still using Eureka made nozzle)
Tri-Star CXL 2 speed now using Tristar made power nozzle,
CXL Gold 50th anniversary
CXL light grey/blueish color
CXL Grey speckled (introduction of crush proof electric hose)
DXL (introduction of greared belt system),
theres one more model here that is still the old style but I for get the model number. After that is all Areus owned EXL MG1 MG2 MG3 and CS