The strangest thing I've discovered so far about my new GEM45 is that although the power plug is indeed 5.5 mm o.d., 2.0 mm i.d., positive center, exactly as specified, some plugs of this size won't go into it, and others will.
I got out my calipers and figured out why. The hole in the iOptron mount's housing is exactly 5.50 mm in diameter.
But American-made power plugs normally run large. Switchcraft and at least one other manufacturer round the size up to 0.218 inch, which is 5.54 mm, and then have a tolerance allowing them to be slightly bigger. And judging by the ones in my parts bin, they always run large, not small.
Asian-made ones, on the other hand, run small, about 5.47 or 5.48 mm. This includes the ones sold by Radio Shack. I think it reflects clearer thinking about how the connector works; it can be a good bit smaller and still make good contact in the socket, but if it's too big, it might not fit through the outer hole (as happened to me).
Now we know. When my mount is out of warranty, I may ream out the hole a tiny bit. In the meantime I know why the plugs I use in my Celestron will not fit my iOptron even though the specified size is the same.