ALASKANFIRE. The adapter you are inquiring about only came with the Tac-13. I searched the planet over as well as a couple other planets coming up with ZERO.
I inquired with Remington prior to their demise and they did not have a listing or PN for it. Muzap's pic is what that adapter's most obvious use will be. Fits the Shockwave Robinshead/Rapter grip as well.
I think it was designed for that purpose only but Certain 870 stocks will work well on it.
I hope to solve the issue very soon. I hope to have the prototype completed and in the final stages of a small run production of around 100 pcs. If those go out quickly, I will do more.
The biggest issue with the part is that it has so many angles and steps to create. Not a good good candidate for CNC work. Cast maybe being the best as I think the original was cast.
Muzap's pic really show how well the Magpul SGA stock really works with the V3. I have already completed an 11 inch V3 using my one and only Tac-13 adapter with the SGA stock for the 870.
It's since become a folder with a different adapter, different Magpul buttstock, and it looks great and functions just as well. I'll post again when I get some adapters completed.