They all say 1 of 300? That's very strange.
My Colt is 249 of 250 in it's series. I would have thought they were all done that way, it certainly makes more sense to me.
Did it come with the original paperwork? Is there any information as to who did the upgrade work? There ought to be some card or something to describe it's background. My Colt was done by an outfit called America Remembers. If you can figure out who did it, you can call them. If Remington had it done "in house" then you might be best served calling them for details. In fact you might be best served calling them first, they should be able to get you pointed in the right direction.