We've had lots of threads regarding sleeving. Pete Mikalajunas, Raimey and Toby Barclay have provided much of the information.
Unfortunately some of the images and links have been lost to time and the photobucket fiasco.
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=224372Toby's very helpful images here are gone
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=177741Here is one I saved
Toby states here that sleeving started in the 50s, but I have not found by whom, nor who first used the term "sleeving" in reference to shotgun barrels
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=561115Another helpful thread
https://www.sportingshooter.co.uk/f...-demibloc-and-monobloc-barrels-1-4766158The process appears to have originated with Henri Pieper's "Diana Breech" patent of August 23, 1881
The tubes were inserted from the REAR of the breech;
Forest & Stream 1882
Leaving a significant step from the breech to the barrels