The most obvious difference is length. The B/M extends 4.72", while the SV extends 8".
But there may be another crucial factor. I can't tell for sure from the B&H photos, or others I've found online, whether the SV has a hex connector (adjustable in 60º increments) or round connector (like the B/M, infinitely adjustable). I
think it has a hex connector. See attached photo. Note the bottom-most section (green arrow) goes deep into the superclamp slot and is secured by the tightening screw. The silver area above it (red arrow)
also slots into the SC's hole and determines whether or not the extension's rotation is infinitely adjustable. This portion of the connector (red arrow) on the B/M 203 is round, allowing for infinite rotational adjustment. I
think this section is hex on the SV 203, but I can't quite tell from the low-res pic.
Here's a link to the
B/M 042 (updated previous post with the link, too). You can see very clearly in the larger image the hex area. Comparing the 042 image v. the B/M 203 image and hopefully it's clear why the 203 offers infinite adjustment while the 042 does not.