The Mark II Airborne Lifeboat was six feet longer than the Mark I, taking advantage of the larger Warwick. Although fundamentally the same conceptually it had larger carrying capacity and incorporated detail improvements from experience with the Mark I. One of the most obvious was to place the propeller in a tunnel. This increased ground clearance under the aircraft on the ground and meant the propeller was less likely to become fouled by the rescue and parachute lines in the water when it was dropped.
Later in the war some Lancaster aircraft were assigned to Air Sea Rescue and the Mark II lifeboat was modified to fit. As for the Mark I modifications to fit the Warwick it was only the gunwale (top edge) that was changed for the different fuselage shape. Indeed all of the Mark IIA lifeboats were initially built as Mark II.