If you get a double stamped body on any rolling stock, it is a different story for value, and that is to someone that would want it, and be willing to pay a premium to own it. Just my opinion.
At one time, there was definitely a market for Lionel factory errors, including double stamped decorations on locos and rolling stock. I picked up a few pieces before the market developed, and have had an interest in that sort of stuff for a long time. The last item I purchased, just a couple of years ago, was a double stamped Rock Island 6464 boxcar. I don't think I paid a premium for it. In fact, I thought the seller had it out at a rather low price, probably because he thought it was defective.
I think the double stamped insulation plate on that 681 is interesting. If I saw it on an engine that was reasonably priced, I would probably buy it, even though I am not looking for a 681. But I would not pay a premium. If I saw the part alone in a bin, I'd probably buy it and put in my oddities collection.
