True all London gunmakers had customers they catered to. If they wanted 27 they would be more than happy to. There was a time when gunmakers would make up a few guns and they did so in the 29 length. It was called "The London Length". That way if a customer came into the shop and wanted a 28" gun or one came in and wanted a 30" gun the salesman there in the shop could most likely talk them into the 29" gun.
As far as gunmakers catering to their clients, I once read an article that Donald Dallas wrote that a client went into Boss & Co. and asked for a self opener. In 1909 Boss did not make that gun. So down the street to Purdey they went and bought an action. They took it back and engraved it in the Boss pattern. Hence a Boss that was actually a Purdey.
