"Why would you want a P-51C? It's exactly like the P-51B, just made at a different factory."
True, but there were differences between subvariants of the P-51B/C which might making a late model useful to have in the MA (our P-51B is a fairly early one, a P-51B-7-NA I think). From the P-51C-5 and P-51B-15 onwards, a different engine was used which improved performance at lower altitudes. While not necessarily viewed as an improvement by pilots (it had the side effect of reducing performance in the all-important 25K plus range), such a variant might be useful in the AH MA.
It is not unlike how different models of the Me-109G had different engines.
That is just one reason why he might want a different version of P-51B/C.
J_A_B