Hi Dan -
Yes - this thread is coming to an end. Because there's not much left to say. To be sure & make yourself happy, you would need to send it in for a cert. We can tell you that with 99.99% probability, it will come back as a SL watermarked #439.
There is no price for a used 476A - because - to date - there are no used copies of 476A. Check out this link - and especially the description at the end of the 1st stamp:https://siegelauctions.com/lot_grd....tFirst=First
You'll see that Siegel has 93 entries for 476A - and none of them are used!
You're talking about one of the rarest - and most controversial - of all US stamps.
The stamp can certainly be certified - as many others have been. Detecting watermarks - especially on orange or yellow stamps - is a challenge. One needs advanced technology, which most of us do not possess.
Scott has no fault here. Once a used copy is certified & sold, Scott will list a used price.
In the meantime, I'd suggest a couple things. Give it a few days & then look at the stamp again. You're holding it - and pictures online are inadequate compared to having the stamp in hand. Try getting a high-res scan of the stamp in fluid. And then, maybe run the scan through retro-reveal & see if anything appears. If you're still convinced at that point, send it off for a cert. Just don't forget to come back & tell is what happened!