Steelie

the jewel of his collection, a steelie from 1943

the heaviest in his collection, an old steelie from the junkyard

The steelies were just balls of steel, industrially ugly and not part of most girls' collections. There were four sizes of marbles and we called them boulders, biggies, regulars, and teenies.

old Mopars wearing steelies with dog dishes

Better wear your steelies when he's around. [implying that he is clumsy]