My mother made me one when I was a skinny teenager because she was worried about my propensity to disappear into the hills on the bike and turn up home drenched and shivering blue in the evening.
It was cotton but pretty waterproof so I assume it must have been Ventile or similar. She modelled it on my older brother’s one.
The real thing was cheap as chips from surplus stores back then.
When I was older I got a real one and wore it on my motorbike. It wasn’t so waterproof on that so I painted it with rubber tyre paint. That did the trick, but then it stank. It was better than my Barbour jacket though.
My Buffalo reminds me of it – without the stink, and far far better.