Hi there,
On my '82 300D and my '79 450SL, there is an interlock built in so that if the engine doesn't start on the first turn of the key to the start position, the key must be turned back to the off position before the starter can be engaged again. I don't remember if the '75 280C has this feature or not...
I think the reason is to prevent possible damage to the ring gear.

Regards,
Richard Wooldridge
'82 300D/T700R4
'79 450SL
'75 280C