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chune - Welcome to the C&V Forum.
Good question. I have wondered that myself.
the other consideration is the distance between the mount and the pivot. My wifes 74 Raleight GP Mixte has a Simplex Prestige with a longer distance than the Vx GT which I wanted to use. It will not fit under the largest sprocket. I tried a different hanger that dropped the RD a bit but still couldnt' make it work with the original block.