I intended "operator error" to mean shooting the gun when it is not properly assembled. That is, either missing the washer or a loose stock bolt.
The proper part name for the "washer" is actually "stock bearing plate" and can easily be found if you google "870 parts diagram". It fits bewteen the stock and receiver.
The circumference of the stock where it mates with the receiver is not intended to be a load-bearing surface. It is undercut and should not make contact with the receiver. The special washer (bearing plate) is used for 2 reasons:
1. To achieve the proper clearance around the circumference.
2. To provide a larger surface area for the recoil force to be transmitted to the stock. If the stock only contacts the receiver around the narrow undercut circumference, it WILL split.
I suggest checking to be sure that you have a small (nearly paper-thin)gap and no contact around the circumference. If no gap, remove the stock and check whether the bearing plate is missing. You can order them from Midway, or maybe Numrich's.
This bearing plate is also a feature on the Sportsman 58 which can exchange stocks with the 870. I have 4 Remington shotguns from 1956 - 1959. I know the 50-year history of 2 these guns because my Dad and I shot them weekly for doves and skeet with our own reloads. Of course the stocks do not look new, but there are no cracks or chips.
Looking more closely your photo of the older 870, I wonder if that crack could be from a lateral force -- such as sitting on the gun in the car.