The cars were originally supposed to have the yaw dampers on them. The bracket on the car body has always been there, on all M-7 cars.
The manufacturer removed the dampers from the design, for whatever reason. Luckily, they kept manufacturing the cars with the brackets on the car bodies.
Then, there was a problem with the 750 VDC buss between the shoe fuse boxes on either side of the truck. It was a solid buss bar type of thing that was fatiguing and breaking, from what I can recall. They changed the design to move the shoe fuse box off to the side, with more forgiving flexible cabling. But the cars were still not retrofitted back with yaw dampers at that time.
Then, the yaw dampers were added back on, although apparently with a slightly different mounting arrangement than the original yaw damper design. There is some welding on the trucks involved with the latest mod, so the trucks need to be changed out again. AFAIK, it is all covered under warranty.
The lower numbered cars, with the original setup, will of course receive trucks only once, that have all the modifications done on them.
So, technically, there are at least three differnt types of truck configurations running around the LIRR.
That's the best I can do with that for now. If I can find out the exact particulars, I'll explain it better. But, few people here likely know the particulars, to tell you the truth. Up in Equipment Engineering, somebody knows. At our level, it's gonna be tough to find someone that knows the whole story, but I'll try. There's one knowledgeable guy that the LIRR raided and hired from BBD. He may know.
Tom