According to Ian Skennertonicon:"Obsolete and downgraded Lee-Metford, Lee-Enfield and short Lee-Enfield rifles the primary source for service small-bore traing conversions were after the Morris and Aiming tubes became obsolete. From 1907, a series of long and short Magazine Lee-Enfield rifles was adopted for training which utilized soldered-in barrel tubes and chambered the 22 rimfire cartridge. Some such conversions by the trade may be found marked 'Parkerfield' around the crown of the re-crowned muzzle."
More can be found in "Britishicon Empire Cadet & Training Rifles" by Ian Slennerton
found at www.skennerton.com