Anchor plates for U-bolts that hold rear leaf springs to axle housing (why are left and right sides needed) shocks do not mount to anchor plates

Hi everyone,
This is something I'm just curious about since it does not make sense.
The four U-bolts that hold the rear leaf springs to the axle housing
on a 91 Chevy K1500 pickup, work with removable U-bolt anchor plates that go under the axle housing. There is one anchor plate on each side of the vehicle. Each anchor plate works with two U-bolts. The shocks and/or shock-mounts are not connected to the anchor plates (like on some Dodge trucks I've seen), and nothing else is mounted to the anchor plates.
For some reason, there are two different part numbers for the anchor plates, i.e., left and right sides for the drivers and passenger side of the vehicle. However, both parts look identical. I held the parts next to each other, and I looked at the parts comparing all the features and they looked exactly the same. I measured the parts with a tape measure and all the features and hole spacing of both parts measured the same.
Since there are two different part numbers it seems there must be some difference between the parts but I cannot detect the difference by looking and measuring.
Can anyone tell me why left and right side U-bolt anchor plates are necessary for this vehicle ? What is the difference between the left and right side parts ?
Any feedback would be appreciated.
Thanks John
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