The Nylfor 'plastic' headsets were imported by Holdsworthy and first
listed in 1967/68 - see image below. Sadly I dont have a top locknut
for the black set. In my experience of these, I found they lasted well
for what they were. Many nuts were probably destroyed by tightening
with Mole Grips etc.
Below is page from Aids to Happy
Cycling 1967/68 showing the first
mention of the Nylfor headset