In searching for recent sales on this item, none appear.
Having clear photos of the gun and (if present) the original box and contents would help with determining a value. I would guess just the gun in average condition would be $1100-1200.
I have been watching 3" CS-1s for some time on GB. I have seen prices from $1500 to $2500 depending on condition, box & papers, and bidders. I would have bid on the one that went for $1500, but I got distracted and missed the end of the auction.
Rumor has it that the USG contract guns went through the M modification or inspection twice. That stated, I can recall at least one example with the stamp that did not letter as a Customs Service gun.