.22 Short Rifle Mk. IV or Rifle No. 2 Mk. IV*.
I converted a No. 4 rifle to .22 (like a No. 9 rifle) by machining a piece of .303 barrel to the dimensions of a .303 case, leaving it a bit longer overall at the breech end, and bonding it into the chamber of a .303 barrel. I then ran my liner drill through, end to end. Liner was then installed. No reason this could not be done with a .303 SMLE barrel to create a .22 SMLE barrel. Parker Hale certainly sleeved many. A .303 bolthead can be converted, as can the firing pin. This would not be a minor restoration, starting with just the receiver, but it could be done.