Speedo / Tach / Gauges: Speedometer screw?

[over50]

On my 1950 2R5 speedometer, where the cable screws on the back, there is a screw looking piece sticking out the side (7:30 position). It is maybe an inch long with serrations along the outside and a slot for a flat blade screwdriver in the end. I've looked through all my manuals but couldn't find any mention of this. What is it for?

Thanks
Norm

[gordr]

It might be a plug to fill a lubrication port. Most speedometers have a boss somewhere on the cable stem with a small brass Welch plug in it, and behind that Welch plug is an oil-filled wick. Possibly somebody removed the plug, added oil and screwed in some plug that would fit the hole?