Ian,
From the 1925 OB&OW handbook:
“The induction pipe joint is of special patented design. Where the induction pipe enters the manifold, an annular groove is formed on the inside of the casting, which accommodates a piece of oiled worsted, that is threaded through communicating holes in the manifold and induction pipe. By turning the induction pipe to the right, the worsted is caused to fill the groove, thus providing a flexible, air-tight joint.”

The illustration below should make all clear. This same method was used on induction pipes for the WW1 rotary engines (example: Bentley BR2), though there they used a ribbon-like material lead in through a slot. The end of the ribbon was stuck to the tube with a daub of Seccotine, some sort of glue, so that it would be drawn in as the tube was turned rather than just slipping.

-Doug