B31 steering assembly

[B31Pete]

Greetings.
I'm trying to assemble the steering on my B31, but there is something missing.  There is no steering damper, and there has never been one of those twirly knobs on top of the yoke.
The bike was stripped down far too many years ago, and clearly something has got lost, but nobody recognises the one component that I do have.
What I have is a stepped cylinder that fits snugly in the bottom of yoke down tube, with a threaded section that pokes out of the bottom of the tube.  There is a screwdriver slot both ends, and I imagine that this simply replaces the the long spindle the the twirly bit normally sits on.  What I don't know is, what should this threaded section attach to beneath the yoke?  Is it simply what would normally go on the twirly knob type damper?
Can anybody shed light on this?  If I know what I'm looking for, or what I have to buy, I will be able to make progress - rather than putting off the rebuild for yet another year.
Thanks in anticipation.
Peter

[JulianS]

The service bulletin and part of a page from parts book for 1954-57 may help explain.

[B31Pete]

That looks very promising.  Thank you very much