In Spring 1943 a much improved version of the Mk IX was introduced. There were two variants to match conflicting requirements. The high-altitude intercepter, Merlin 70, had an optimum ceiling roughly the same as the original Mk IXs. As a result this variant did not enter production until 1944 in rather limited numbers. Officially known as HF.IX.
The low-level fighter with Merlin 66 optimised for altitudes lower than the previous 60-series entered production by mid-1943. This would become the successful, mass-produced Mk IX. When first produced it kept its original Mk.IX designation. In order to tell the difference the crews called it Mk.IXB. To avoid confusion the Air Ministry introduced the designation LF.IX for the MK.IXB while the original IX became the F.IX. Although LF suggests a low-altitude fighter it is not quite so as it developed its optimum performance at 22,000ft. (6,700).