My 1966 Land Rover and 2018 Roadster both had the tapered wedge controlled by Phillips screw that locks arm onto splines, as Jon describes. I did replace the arms on my Roadster with superior black Caterham ones though. These have a spring catch that locks arm onto splines. The tab of this catch needs to be pushed to release using a flat blade.
Now I have no idea if this latter type were ever used as OE on Morgans, but if so or as retro fit like mine I think you could do damage if you just tried pulling them off without releasing spring catch first.
