Interesting thoughts guys. I can't say I've heard of a left hand thread on such applications, as they are usually only used when a right hand thread would become loose during use - say on certain circular saw arbours. There's nothing to say that they didn't enjoy the idea of being different though! It may be such that the two surfaces have essentially grown together. I'm toying with the thought of running around the edge of the wood that meets the metallic washer with a very fine kerf saw down to the tenon to both remove that locking face and allow somewhere for more penetrative agents to ingress. It would alter where the stem sits once tight of course, but I can worry about that hurdle when it comes time to jump it.