Funny shaped nut

Looks like a wheel spindle nut to me, with the rounded shoulder to accommodate the adjustment bolt.

No it isn't correct. A normal flat nut is usually fitted so it pulls up tight on the washer. 7/16" x 20 tpi is correct.

Jonathan,

Good to see that it is not just my bike which has this nut in this location.

Jonathan..........I have seen a few frames, like yours, with sidecars attached. Some had the usual flat nut replaced by the more substantial wheel nut kind.

I agree with that - with a sidecar attached a large fishplate fits over the left-hand swinging arm gusset plate. The shouldered nuts protrude through the fish plate on to the swinging arm spindle and the top engine plate mounting tube stud. Strangely, the later frames did not have the top stud hole in the gusset plate as the tube was welded - so presumably to fit the sidecar fishplate would have required some careful drilling ? Someone somewhere has got your "sidecar type" swinging arm spindle nut. Cheers, Jonathan and Phil. Regards, Howard

Fascinating stuff, my bike had a sidecar in an earlier life so that all makes sense then.