Colt did manufacture a run of SP1s in Coltguard, primarily for the marine trade (assuming stronger resistance to salt-water corrosion on the steel parts). It resembles a matte nickel finish. I don't know exactly how many were made; well under 1,000, I believe. You could always get one of those pricey Colt historical letters if you really needed to know the numbers.
I picked one up, used, for $200 from a cash-starved buddy in the early '80s but it drew too much attention from the gold-chain crowd. I ended up selling it a year or two later for $675 to one of the aforementioned gold-chainers.
There were one or two for sale on the boards just before Y2K, with asking prices in the $1,600-$1,800 range. I don't know if they sold, or for how much.
If yours is an original and in collector-quality condition, I would guess it's worth $200 to $500 above regular SP1s in the same shape.