Here is a link to a recent listing on GB. It has already sold. https://www.gunbroker.com/item/969649225
I really thought about trying to buy this, but there are two or three things in the listing that I couldn't get around. The price went too high for me to risk a gamble.
First, it states that it was originally built as a 20 gauge bore rifle, which really raised my pulse. They said that it was confirmed with by a conversation with Purdey that they built the gun that way but it had subsequently been rebored to a 410 gauge.'
L,S&B are generally honest and accurate, but this bothered me. It is obviously impossible to bore a 20 gauge to 410. Also, they didn't say that Purdey did the work, but I doubt Purdey would offer the opinion unless they did the work. There is no indication the bores have been lined.
They also were of the opinion that the firing pins were off-center and would not fire a striker of a 410 cartridge. If it was buile as a centerfire, no matter how it arrived as a 410, that would not be true.
All in all, I would loved to have the gun and tried to obtain tubes and lined to the original configuration. Rifled 20 gauge tubes are available out there, and this would have been a very nice gun. I already have a Sauer Cape Gun thet is a rifled short 20 on one side. The wood, engraving etc on this gun is really nice and it already has what looks to be a dovetail for a rear sight.