You are correct, odd calibers and/or features (such as the under-lever) can be difficult to move (certainly Lewis Drake, if anyone, knows how to price a such vintage piece). But, if a person wants a true best-quality gun (not one merely pretending to be), you will not find a better deal. In this regard, the rifle is an absolute steal.
It reminds me of a Purdey 12 bore I saw at a gun show once upon a time (roughly 20 years ago). The serial number placed it in the early 1920's, and it of course had all the lovely features common to a Purdey (the Beesley action, Wem ejector (basically a Southgate), Purdey's house engraving, etc.). The gun was as tight as new, firm on the face and in lovely condition, and the guy was asking only $10,000! This is for a Purdey made at a time when, many would argue, they were turning out the finest of the fine. It was well beyond what I could afford then (still would be) but I asked the guy about the shotgun, and the price. His comment was that it had 2-1/2" chambers so, in his mind, it was useless! I was dumbstruck.