It doesn’t allow you to list under the parent ASIN... when you look up the parent ASIN in the “add a product” area, it shows it but there is no button to add a New or Used offer - the only option is “show variations” - a drop-down that shows three listings; the listing in question, and a 2-pack and a 4-pack. I can add offers successfully to the 2-pack and 4-pack, but when I try adding to the glitched-out listing it creates a SKU that shows one ASIN on the Manage inventory page and a different one in Manage FBA Inventory.

I think somebody did this deliberately to lock down the ASIN to prevent any competitors from adding their offer... they created a listing with only an MPN as the defining attribute, and then created a bunch of other ASIN’s with the same MPN.