Drew, earliest Proof book mentioning it that I have is dated 1960 and referred to the notes being an up-date from the 1956 book. In it it mentions that the process was brought to the attention of the Birmingham Proof House in 1955. doing a bit more digging in my library I find the following excerpt from the minutes book for 1955. 'During February the Guardians became aware of the first two cases of re-sleeving of Shotgun barrels being undertaken by a London company. The process involved cutting off the old re-useable chambers of a shotgun, making two new tubes and sleeving these into the old chamber section using solder. The Guardians initial reaction was to tell the Proof Master to refuse to prove them and in the interim they decided to get a specimen and subject it to a 20,000 standard service cartridge test. in March the London Proof House agreed with their actions. Shortly afterwards they were advised by Messrs Charles Hellis that this had been common practice in France Belgium and Italy for years. In July the London Court reversed their former decision but Birmingham continued to apply their ban until the test results were known. Finally in January 1956 they agreed to rescind their decision following advice from the Law Clerk, who that their actions were in contravention of the Proof Act.' The excerpt was taken from 'A Bi-Centenary History of the Birmingham Proof House 1813 - 2013.' by C.W.Harding. Hope that helps a little. Lagopus…..