Complex sentence: This was a permanent measure designed to stop in-flight rear fuselage structural failures and was introduced on the production line from the 820th production aircraft: between December 1942 and March 1943 all Typhoons without "Mod 286" were taken out of service and modified.
Split into simpler sentences: This was a permanent measure designed to stop in-flight rear fuselage structural failures. From December 1942 through to March 1943 all Typhoons without this modification were taken out of service and modified.