Chao's steel calculation is spot on. A 10 or 12.5lb plate is in the 8 1/2 to 9" diameter range, and they do look to be 11" or so in length, so that would be about a #200lb cylinder per side. Lets say a 30" 1 1/16 or 1 1/8" rod for the actual bar, and you're going to be real close to the #420.
Steel is a bit more dense than cast iron, for instance a similarly size 9" x 11" cylinder in cast iron is about #180lbs. Also depending on the design, plate loaded dumbbells can have lots of dead space.