Thanks, Buck Stinson. Your muzzle dimensions are close to what Pirkle gave for the 1st model '73, and I can believe there was some variation at the factory. Indeed it doesn't seem like much, but the cross sectional area (and the weight) varies with the square of the diameter. The "Winchester" rifles by Miroku have an even faster taper, and they are very lively handling.
As I recall, Madis wrote the half magazine models usually had the fast taper, and the photos I've seen over the years (auction houses, Merz, etc) look as if this is so.
Pirkle also mentions a heavy contour besides the regular and fast.
nativeshootist, I think Cimarron just passes your order through to the custom shop at Uberti. Taylor's will do the same, but the available options are limited, and a half mag isn't one of the current options.
Coffinmaker, Madis said the factory would later just plug the normal, stamped forend cap, but the early ones I think he wrote (I don't have a copy) were cast, and the photos show a different contour. One worry in trying to plug the hole would be getting a matching brass/bronze, since the color varies some with alloys, and I don't know what Uberti is using.