The Model 21 also has a special frame for the vent rib barrels, like Walt describes the Ithaca NID. I think the RBL with vent rib is going to be sold at a premium on the resale market because it will be scarce. The vent rib was originally designed to cure heat wave distraction on guns used for ATA trap. The business about not seeing the barrels is a secondary feature, but important to some shooters. I kind of like them but wouldn't pay for one that sat low on the barrels. Tony even puts high vent ribs on his .410 bore guns.
