Re: FN SPR bottom metal
Yes, the parts are interchangeable between the DBM and hinged floorplate.
For the hinged floorplate you will need to roundup a follower, spring, magazine box, trigger guard and floorplate unless you go with the Williams one piece trigger guard/floorplate.
As Turbo said above, the inlet is completely different between the two. If I were doing the conversion I would fill the old inletting in and start over for the one piece floorplate.
You should be able to get a follower, spring and mag box from FN. I would buy the one piece floorplate directly from Willams and save a few bucks. Also, you can give Numrich a call and see what they have for parts.
Hope this helps.