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The main difference is on each end one of the parts has a slightly larger hole to create a loose fitting hinge (about 0.2mm additional clearance). The "B" parts are just a mirror image of the "A" parts. Reason for making the A/B versions is otherwise I was concerned that the assembly may slide apart - have added an image that hopefully explains.
All that said - more than happy to take your guidance as this is all a bit trial-and-error - its greatly appreciated!