It is interesting that the OP's broken piece is from the front of the forend where the magazine cap screws on. These guns are notorious for splitting the rear of the forend (i just finished repairing the forend of my Dad's Remington Model 11 which is an Auto-5 clone) but I think it is unusual for the front to split.
There are a lot of auto 5's around here and I've seen probably almost two dozen broken the same way as the OP's. It's just something they do.