Response to Mt Omeara’s counterpoint that other persons on the 6th floor kept their suits and ties on despite the theoretical heat gain on the 6th floor:
They were government officials and cops. They were required to maintain their uniform appearance regardless the heat factor.
Mr Lovelady , however, was a warehouse worker without such rigorous requirements for dress appearance.
( note : I think Mr Ford alluded to this point earlier )
If the skinny ex marine vet Oswald often worked in short sleeve T-shirt only, it’s not implausible that the chubbier Lovelady wearing a long sleeve undershirt would remove his outer flannel shirt also to be more comfortable on the hotter upper 6th floor.
