The 1968 version of the tropical combat boot retained the integral spike resistant steel insole of its predecessor, but featured mud shedding "Panama" outsoles, rather than the traditional Vibram design.
The Panama sole was conceived by Sgt Raymond Dobie of the Panama Mobile Force, under the supervision of Cresson H. Kearny, in early 1944, though it was not officially tested until 1967.1 The clever design utilized the flexing of the foot to create pressure that squeezed the mud out of the tread, thereby significantly improving traction.
Forty pairs of the Panama sole boot were tested at Tropic Test Centers in the Panama Canal Zone, whilst the 9th Infantry Division tested others in early 1968 as part of the ENSURE program on footwear for inundated areas. The testing verified the self-cleaning properties of the design and on 9th April 1968 Army Material Command directed the Panama sole be used on all Tropical Combat Boots.2
In May 1972 the military specification was revised to permit either direct or injection molded soles and as a result the official nomenclature was changed to Boots, Hot Weather, Men's.31. Jungle Snafus...and Remedies by Cresson H. Kearny (Oregon Institute Of Science And Medicine 1996) (Amazon.com affiliate link)
2. Support To United States Army In Vietnam - Clothing & Organic Materials Vol II (United States Army Material Command Aug 1968)
3. Military Specification MIL-B-43154E (16 May 1972)