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@stanmawr
Hi,
looking at this prompted me to look at my (as yet untouched) Koga project. It would seem I have the same bosses, which is bollocks given that I have sourced some NOS Dura Ace 7400 levers for the other/square type Boss.
Having done a bit of research, it would seem that they are both (contemporaneous) type A bosses but that they are commonly referred to as Type A/M5 (square which in 7400 anyway had a light alloy inner curved washer and always takes an M5 lever bolt) and type A/M4.5 (keyhole which in 7400 had a plastic inner curved washer and always takes an M4.5 lever bolt).
In terms of shimano model nomenclature the difference is SL-nnnn-FCAI (square) vs SL-nnnn-FAI (keyhole) see - http://velobase.com/ViewComponent.aspx?ID=40e79148-190e-440a-8cab-4bbb67c4ecad
Doesn't solve the problem but maybe gives a direction.