As of Tuesday this was for sale on one of the US auction sites.
It had been sold previously and returned as when the barrel was measured something like only the last couple of inches was 410 calibre, the chamber size was wrong and rest of the barrel measured at somewhere around .318 (if I remember). It was / is being sold 'as is with no return'

The WS allegedly is for "Wormwood Scrubs"

Just as an 'add-on' my local gunshop ( a few weeks ago) had a No4: 410 calbre for sale but didnt notice any particular markings.
It did NOT have the magazine well welded up, so as far as the Police are concerned it should be a Firearm (FAC needed) but it was being sold as a shotgun (SGC needed)