P-51L and P-51M
From Wiki (yes I know)...
These included the P-51L, similar to the P-51H but utilizing the 2,270 hp (1,690 kW) V-1650-11 engine, which was never built; and its Dallas-built version, the P-51M, or NA-124, which utilized the V-1650-9A engine lacking water injection and therefore rated for lower maximum power, of which one was built out of the original 1629 ordered, serial number 45-11743.
North American's Lightweight Mustangs (P-51H/P-51M/P-51L)
So it appears to be a Dallas built version...?