oil reservoir to the front end of the recuperator to maintain the oil supply and extend spring life. This modification is visible in many photographs of 18-pounders in action on the Western Front right up to the end of the war Mk I* and II Carriage The recuperator spring problem was rectified with the new Mk II carriage officially introduced in the field in November 1916 with a hydro-pneumatic recuperator design which replaced the recuperator springs with a system driven by air compression and could be fitted into the existing spring housing by battery officers in the field. It is identified by the torpedo-shaped extension on the recuperator, which made the recuperator assembly nearly as long as the barrel and hence altered the equipment profile. Converted existing carriages were designated Mk I*. The Mk II carriage also incorporated a longer cradle. In about 1917, all 18-pounders began to be fitted with a new calibrating range scale. This allowed the gun's muzzle velocity to be set on it and automatically corrected the range for the difference between the actual muzzle velocity and the standard one. Mk III gun on High-angle Carriage An experimental Mk III gun was developed in 1916. It had a semi-automatic horizontal sliding block breech, with the recoil mechanism below rather than above the gun barrel. The high-angle mounting may have been an experimental dual anti-aircraft and field carriage. The design did not enter service. Mk IV Gun on Mk III and Mk IV Carriage A major variant (essentially, a new artillery piece with the same ballistics) was the Mark IV gun on Mk IV carriage. The Mk IV gun started testing in 1916 mounted on a new Mk III carriage with box trail, then typical for howitzers. This eliminated the original central pole trail, which had restricted elevation, allowing increased elevation to 37.5 degrees and hence increased maximum range from 6525 to 9300 yards with the 2 charge shell. Mk III carriage was quickly superseded by Mk IV carriage as the standard field carriage. Mk III and IV carriages incorporated a new variable-recoil hydro-pneumatic system and moved from above to below the gun barrel. The new single-motion "Asbury breech" allowed for higher rates of fire and a Welin tapered screw adopted for the breech. A single battery of the Mk IV gun on the early Mk III carriage was serving with 4th Army when the First World