Thanks guys - the piece of brass does indeed appear to be part of item 21. When first glancing at the third motion shaft assembly it wasn't noticeable, but when peering between 2nd and 3rd gear there's definitely a chunk missing out of item 21.
Also I managed to remove the first motion shaft by pressing it backwards into the gearbox, although it was necessary to nudge the retaining nut with an angle grinder to create a 'jimp', into which an old chisel could be engaged.
My next question is how to separate 2nd and 3rd gears on the third motion shaft. The Haynes manual advises that each pulls off in a different direction, and it also warns about the need to make provision to catch the detent springs and ball bearings. It states that the first step is to press out the collar on the front of 3rd gear. But how? It won't lever out with a screwdriver, and I assume it doesn't push inwards and twist if is resting fully home against the gear wheel itself. Am I looking at the correct part, ie the first item beyond the visible shaft splines on this photo? Any advice?[frame]

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