I think there's some confusion about old model vs. new model .357 Cylinders.
1. New model Blackhawk .357 frames and therefore cylinders are longer than old models because they're built on the large frame. The photo above shows the difference.
2. However Old Model .357s are built on the mid-frame as we all know. But the New model FLATTOP Blackhawks and New Vaquero, except for the 44 Mag, are also built on the same size mid-frame. Therefore the old model .357s (both the flattop and the post 1962 ribbed rear sight style) cylinders do interchange with the NM Flattops and New Vaquero. The overall cyl lengths average 1.835" + or -.
The body of both old and new cylinders is 1.605" plus or minus.
My three new model FLATTOP and New Vaquero .44 Spl cylinders all interchange with my .357 cylinders in both of my old model .357 cylinders, one flattop and one with ribbed rear sight. I don't have any NM FLATTOP or New Vaquero .357s but I can't see any reason why they wouldn't also interchange with the Old Model .357s.