I'm thinking about a Mk 12 Mod 0, with the PRI handguard, ARMS rail, etc. Taggor, that was a nice touch with the NoDak NDS-16A1 lower. I have some of those for my retro builds, as well as the NDS-XM16E1 for a 605 Build.
For those wondering what I'm talking about, when Crane put the SPR rifles together, they used existing M16A1 lower receiver groups with Type "E" M16A1 stocks, but with match triggers installed, with a newly-built upper using an M4 upper receiver, a Douglas select match stainless barrel, and the PRI + ARMS components.
The NightForce 2.5-10x24 SPR scope seems to be a Military model only for the SPR right now, and is extremely difficult for civilians to get.