TR stands for "trafikvagn", swedish for taxicab. The TR701-704 were the successors to the TR671-679 purpose-built taxicabs.
The corresponding standard versions were designated PV656-659. The TR had a longer wheelbase and seven seats. There was a side-valve six-cylinder engine, 'EC', under the bonnet. The Volvo cab was loved by its users and was almost immune to wear and tear.