I can understand why a proof house would be slow to embrace new technologies or new ways of making barrels. The entire point of passing proof is to give the buyer confidence that the gun or barrel is sound and should last under normal usage. Sleeving was a big change from new barrels and I am sure there were questions about how long a soldered seam or welded seam would last. Plus what would cutting and boring a set of old barrels mean for their integrity and long term strength. But technology evolve and testing needs to stay a pace. They were slow to embrace sleeving but did come around.