The first production fuselage tank was installed by Engineering Change order which applied to the 251st P-51B-5NA which became a block change P-51B-7NA. IIRC the same ECO applied to the P-51C-5NT at the same time
All P-51B-1NA, -1NT and the first 250 P-51B-5NA were retrofitted in the field in ETO and MTO. Some RAF Mustang IIIs did not retrofit
Thanks for the clarification Bill - I can never remember, off the top of my head, at which Block the first tanks were fitted, and always seem to think it started with the 'C' model first.