pianolist: I shared your post with the blacksmiths on that forum. Thanks. One other blacksmith noted the Mahler symphony as well, and said there must be over a 1,000 pieces scored with an anvil. It's just seldom done literally. I can see why. It's been years since I listened to the 6th, and I never knew about the anvil. Now I just HAVE to go find it and listen again.
Tim: As you surmised, the Fisher has a cast iron base. It was an American invention and made the US independent from exclusively importing European anvils. As I understand it, Fisher perfected the process of pouring (casting) iron onto a tool steel face. A Fisher anvil is the only quality anvil that doesn't ring. Previously, the top tool-steel plates were always forge-welded onto a wrought iron base. A quality forged anvil was tested for integrity of that forge weld by its ring. Anywhere the plate was not completely welded to the base, it would sound dull and lifeless. So if the plate was successfully forge-welded, it rebounded and rang like a bell. Now, all good anvils are cast or forged from one chunk of high-grade steel because it is easier and cheaper, and they all ring.
There are exceptions that we call: "Anvil-Shaped Objects" or "ASOs". These are cast iron imitations of anvils that worthless for forging. You can find them in Harbor Freight, Grizzly, and various other non-blacksmithing supply houses. They look like anvils, but are actually boat anchors. They do not ring at all, as you noted, being just a hunk of soft cast iron.
I live about 20 miles from New Market VA, and grew up less than 20 miles from Gettysburg, and haven't attended a reenactment yet. But now you got me curious about the sonics of such an event. Maybe I'll go check one out. A coincidence: my home-built double-axle trailer for hauling machinery and other toys, came from someone who used it to haul a reproduction cannon for reenactments. I never saw the cannon.