The firesights and mag extension came in. The firesights are perfect for what I have in mind They are fiber optic rifle type sites that attach to the rib.
The extension I got was a little longer than I anticipated. Once I installed it, the extended tube came to exactly 19 3/4", which falls between two rib attachment points. If I use that tube extension I'll have to cut the barrel to 21" which is a little longer than I wanted and just looks out of proportion with the pistolgripped folder installed.
I think I might order the next shorter tub extension and cut the barrel to 18 3/4" on the one with the folder and use the longer extension on the wood stocked 870 that I keep at 28".