UDT stands for Under Water Demolition Team. The UDT teams were established during WW2 and consisted of especially trained US Navy divers. These divers had the job to clear underwater obstacles for amphibious landings. It was a hard training and these men had to spend a lot of time in the water for getting their special diving skills. During the 1960s the US Navy introduced reversible shirts for distinguishing students from teachers whilst in the training and to indicate dive status. These shirts were mostly private purchase which explains the many variants in color and sleeve lengths. Made from 100% cotton they could be turned inside out from dark blue to bright yellow. Later in the 1960s these shirts could also be seen in use by US Navy SEALS.