Its is registered and should be scanned at every leg of the journey correct? UK does not have a class of Registered Mail that is recognised for special treatment by USPS. USPS registered mail is treated as normal mail in the UK. The nearest UK equivalent is International Signed For which amounts to delivery confirmation but travels with other first class mail once in the USA.
Great coin and I'm happy for you that it arrived! I bet the seller is pleased that it wasn't a loss for them either.
I'm glad to read this had a happy ending. The one coin I had go missing for a couple of months or so surfaced after a postal inquiry as well. USPS is slow but sure apparently.