The emerald of Catherine the Great

Mr. Collop describes the Finesse Diplomatique of Bogotar.
MY DEAR MAURICE:
This is the fourth book I have dedicated to you, and you will see why if you read it—which no one need do.
First, emeralds are green; and, on principle, like the Green Overcoat, it owes to you of the Green Elephant, a dedication. Next, there is Catherine the Great. She plays no long part, but she founded the fortunes of them all; and we are in communion in the matter of that large and generous but regal soul; we agree that it is a pity she died before we were born. Also, you who know all about Russia, and I who know nothing, have, in the matter of Russia, this Monarch of all the Russias for a link.
Lastly, you have often urged me to write a detective story, because (you assured me) they have gigantic sales. I promised you I would, on condition there was nothing to find out.
Here it is.
KING'S LAND,
Whitsun , 1926.
William Bones was a stalwart man, some thirty-five years of age, the master of a Brig which sailed from the port of Boston in Lincolnshire and was half his own property. He was a native of that town, his father having been therein a pork butcher in a fair way of business, his mother the daughter of a small farmer in the Wring Land. He traded with the Baltic when George the Third was King—indeed, when George the Third was still young and long before George the Third first went mad.
Among other ports, he had found profit more than once in visiting that of the River Neva, and was acquainted with the Russian trade. The great city of St. Petersburg, still new but already splendid, became familiar to him; and he himself, in his humble visits to the local factors, became a familiar figure to the Secret Police of that capital. Even his most domestic and private actions during his dealings in this port were registered; and, it must be added, his strong English frame and handsome English face admired, but also duly noted and their description passed on to the proper authorities.