New Equipment on M-7's

I noticed this week that some m-7's have equipment that looks like shock absorbers mounted on the outsides of the wheel-trucks. I remember a while back there were problems with swaying on these cars. Are these devices the fix for that problem? On the train I was looking at, the higher number cars had them, and some of the "older" lower numbered cars did too. So would I be right to guess that the older cars are being retrofitted, and all the "newer" M-7's are being built with them at the factory?

Those are yaw dampers. They help reduce the swaying of the cars.

The newer cars from the factory, as well as the retrofitted cars, have the shocks installed, and the shoe fuse box is moved to the side, pointed down.
The lower numbered cars are indeed being retrofitted with trucks with the mods installed. The unmodified cars are easily identified by the lack of the shocks, and the shoe fuse box in it's original position of centered right over the shoe hanger.

I recall seeing them on the test sets before the M-7s entered service. Then they went away - anyone know why?