The outer cups are anodized aluminum like the 1st silver 325.
One way to differentiate is the cups are larger (longer) than the plastic cups.
Grado never made a large plastic cup until the mushroom shape "i" cups, which were enlarged to match the volume of the 325 cups.
Remember these were John's best headphones when they came out. They pre-date the first RS series.
IMO the larger cups, even more so than the material, are what made these sound better than the lower SR range.
I don't know why they sound better than the silver 325 that came later. Maybe the earlier batches of SR drivers (mine are very pink transitional, were a little different.
I bought 1st generation MS2, which have same cups but not pink drivers, as back-ups. They also look and sound good, but not as detailed and clean as the black 325.
The early metal cups are really nice lookin', to me better than the mushrooms
The inner cup, which hold the drivers, is plastic like the other SRs - so they are not ALL metal like the HP-1 or HP-2.
I always wanted to mod to wood inners, but I'll never touch these. Maybe my MS2 someday....