I've got an outdoor rotary dryer, the top nut or cap has gone, without it, it's difficult to put up. Brand is windshield, there's no thread. Link to photo; https://ibb.co/W6VY6pY
ON teh left of your picture is a toothed washer on a horizontal bar - that is what is missing. Sometimes they are in fancy caps and yours may have been. Go you a car spares store - an independent to the big shed type, and ask there. Take an accurate measuement and you should find one.
Failing that, if it slips again, drill a hole across the top "pin", and use a sideways pin through it to hold a regular washer on to the top. a pin like this through the hole drilled is what i mean: