Jolodo Even in superb condition with crispy new boxes the set is a quite common $300. These promo set were made in large numbers and sold cheap. 7006 sets had 1684 locos.... not 258 locos, so either the set box is wrong or the loco is wrong. If it was a Gunmetal 1684, which is scarce, even rare, double the price to $600.
The C mean the tender had an electric coupler.. the x means something "out of the ordinary", you may have the set box for the gunmetal loco.
I might be interested IF you are wanting to sell the set. You can PM me if you are.
