Originally Posted by
alcjphil
Braze on mounts have a vertical slot that allows the derailleur to be moved up and down to suit chainring size. There is no need for wedges to move the tail of the derailleur up and down, you loosen the bolt that attaches the derailleur to the mount and slide it up or down to the correct height. The mount has a curved channel that allows the derailleur to be pivoted a few degrees so that the cage can be lined up with the chainring
Some vintage deraillers like the 1981 - 1984 Dura Ace AX and 600 AX ones will not follow the curve of a 52 teeth chainring unless a shim is used at the top of the derailler/brazeon interface to lower the tail of the derailler cage.
To correct that it would take either a 1mm shim at the top or a tapering job to remove 1mm from the mount.
Cheers