Something's not adding up here... if it's truly a pre-ban, it wouldn't have the LEO stamp on it - that marking was not required until several months after the ban went into effect.
If it's got a LEO restriction stamp on it, then it was made and shipped after the ban, or was reworked into a LEO-only config by a licensed manufacturer from a stripped lower after the ban (essentially remanufacturing it, and making it a restricted post-ban now).
My questions to you would be how do you know it's verifiably pre-ban (letter from colt, receipt, what?), and if it's verifiable as a pre-ban, then how did it get post-ban SAW LEO markings on it?
As for value, that'll depend on which model number it is and if it's not restricted. Somewhere between $1500 and $2200 for new, unfired with no box, depending on model.
If it's a restricted LEO gun, then whatever a similar new LEO gun goes for today, less something for having no box.