Tim,
I'm not an expert with any gun design, but, there is something a little bit suspicious in the quotes from that book. If one is going to set about copying a design of English sidelock, the Holland & Holland sidelock, with Southgate ejectors, is the easier of the two to pull off. The Beasley patent Purdey self opener is much more difficult to get right. I'll let others argue about which design is superior, but, when one sees a well executed sidelock game gun copy (think current production Spanish or Miroku sidelock) there is good reason that it is the Holland design. Further, almost all of the makers out of Spain today tell you straight up in their catalog that their sidelock guns are paterned after the Holland design.
Why would they have changed, if they spent time and effort to get the Purdey/Beasley copy right?

Best,
Ted