I would agree with Ted that a distinction should be drawn between the monobloc or similar systems as a method of new construction and sleeving as a method of replacing worn out or defective tubes.
In the case of the latter, the method was certainly pioneered, if not invented, by Christopher Ashthorpe the engineer and gunsmith of Seven Stoke in Worcestershire in or about the 1950’s.
He had to overcome considerable reluctance on the part of the Proof house, which finally agreed to proof re-sleeved guns. The Proof house at first insisted on the word “SLEEVED” being stamped on the outside of the breech ends rather than (as now ) on the flats.
This led to Westley Richards, when they started sleeving, making a virtue out of necessity by emblazoning “Westley Richards” boldly on the sides of the barrels above the “SLEEVED” mark to demonstrate their confidence in the process.
I understand that Mr. Ashthorpe did not regard his idea as Patentable, being aware of the earlier monobloc designs.
There was also a long standing practice in this country, at least since World War One , of restoring rifle barrels by Parkerifling them.
Going back to monobloc designs, some early nitro double rifles, by Fraser amongst others, had very thick and somewhat bulbous breech ends. Does anyone know if they were built using a form of monobloc or two piece construction?