In November 1965 the USARV (United States Army, Vietnam) Tropical Combat Uniform Board recommended the removal of the Tropical Combat Coat's epaulets (shoulder loops) and waist adjustment tabs. The board found that the epaulet buttons were prone to dig into an individuals shoulders when wearing a pack and that the loops themselves snagged easily and were a hindrance when moving through the jungle. The rarely used waist tabs were also deemed to be prone to snagging and causing discomfort when lying down or when carrying equipment.1
Rather than being adopted immediately, these two changes appear to have been introduced over a short period time. Some Jungle Jackets were made without the standard 2nd pattern's epaulets but retained the waist tabs, others had the waist tabs and no epaulets, whilst others had neither. However, all these variants on the 2nd pattern jacket did retain the gas flap.
1. Summary of Proceedings of United States Army, Vietnam...Tropical Combat Uniform Board. (22 November 1965)