Looks like 1-1/2"x20 55* whitworth?I've a vague idea that they may be 1-3/8" 20tpi, but it's been a long time since I removed any and had the correct extractor. There are at least two different sizes too.
A quick Goggle suggests 1-1/2" x 24tpi. Draganfly did stock them. But Burton Bike Bits might be a worth a go, if they still exist.
It doesnt macth anythink else this time of nightI'd thought 20tpi, since it's a standard thread. Definitely 55° Whit. I think that !-3/8" might be for the C and early unit B models.
Cheers.