Arne Jacobsen office chair - Five Wheels??  

Can't provide them to employees in the US with fewer
than five casters. OSHA regs.

And similar regs in Europe,
many of which predate the US rules.
Bases were changing from 4-star to 5-star throughout the 70s and 80s; I don't know exactly when the change occurred for the 3117/3217, but it had probably happened by 1990. My 1999 3217s have 5-star bases and they weren't remarkable at the time.

always the same answer
josh,
Do not waste your time asking for speculative answers (and opinions) when you could contact Fritz Hansen and get "the answer". It is too easy to go the manufacturer.

A quick aside for tktoo, if...
A quick aside for tktoo, if you havn't seen it on the FWW site check out whats happening with safety regs and the sawstop, good reading.
http://www.finewoodworking.com/item/41527/appeals-court-upholds-osorio-t...

Fritz Hansen says:
For anyone that is interested, Fritz Hansen has confirmed that the switch to five wheels happened in 1975, in conformance with new 'DIN' norms.

the answer
josh,
I am pleased that you contacted Fritz Hansen. This forum produced one good guess and the manufacturer provided one correct answer. Congratulations.

Thanks, Heath.
I purchased a SawStop machine for a previous employer and used it often. Serious saw, but finicky blade changes.