The Rem 700 SS Mil-spec was a special build done by Remington for the US Army. It is not a 700P, PSS, or LTR.
The SS Mil-spec has a 24" 5R barrel with 1 in 11.25 twist. It was designed to shoot the 172gr HPBT .308 Win. The 5R barrel has 5 lands as opposed to 6 lands. The PSS and LTR have a 1 in 12 twist 6-groove factory barrels. The 5R barrels was developed by Boots Obermeyer and exhibits improved accuracy and velocity over the standard 6-groove.
Less than 500 were released to dealers, the rest were sent to the US Army for M24/SWS evaluation.
PSS - 26" barrel 6-groove 1 in 12 (308 win)
LTR - 20" barrel 6-groove 1 in 12 (308 win)
SS Mil-spec - 24" barrel 5-groove 1 in 11.25
So yes it is a rare rifle variation on the Remington 700. I've seen them go from $900 - $1200 for the rifle alone.