I see spare road wheels stored on the top of the turret, port side. They aren't mounted to the manhole cover, as these M1's didn't have that yet.
I know that you can use a track tooth to clamp through the hole and around railing/basket as shown here (although I think the tooth should have gone perpendicular to the railing):
However, I don't know of any fixture to secure a road wheel too on the upper part of the M1 turret other than there's four bolts there. I can't imagine removing one of the bolts to put a longer one in and use for mounting.
I've only been personally intimate with the old M60A1's and never had hands on experience with the Abrams. Any info on how the thing is secured there would be great!