The DSP73 form is issued by the Department of State (Dept of State Permit) and allows the temporary export from the USA and re-entry of military items but does not cover duty relief procedures. Also, a DSP73 form does not exempt the UK company from complying with UK export control regulations for military goods so the goods must be returned after repair under an export licence - for the USA this will probably be OGEL Military Goods After Repair but you would have to check the TsCs of the licence to be sure. Further information can be found on the BIS website in the Export Control section.