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IIRC that is the ADL version of the 878. Most of these had vent ribs also. All 878 no 58 parts used. Stock and forearm on this version also was checkered. The safety on yours is not Remington.
Sorry cant offer help but will follow this as I just got one myself yesterday. I was reading on google about how 58 parts fit the 878 but cant offer proof. I did see talk about 870 barrels working but required drilling gas ports