Now fortunately the body of that assembly was still in place, and unscrewing it I deduced that it must have a plunger to block the inner orifice, a compression spring, and an adjusting bolt with locking nut. Absolutely no idea what sort of strength the spring needed to be, but in the event the choice was very limited in my spring box.
Internal thread turned out to be 1/4" BSP / 19 tpi so I steamed up the lathe and set too and made these: