Hi,
The idler gear and the shaft it fits to are separate probably for ease of manufacture and although a tight fit the key is to ensure no movement that could occur. Any movement would upset the valve timing and the ignition timing as the points are attached to the idler shaft.
The 1970 parts book shows the pinion and shaft as one part number, 70-9104 and a key as part number 27-1386.
The 1965 parts book is similar but the keyway is shown in the illustration.
I would imagine that really only having one part number would imply that the idler pinion, shaft and key would always be treated as one component.
Dave.