The -73 engine is not listed in most of the sources on the Allison engines for the Mustang. Like "Vee's for Victory".I also think that the British got some early-ish Mustangs (don't remember the precise mark) which had V-1710-73, which is the same engine as the P-40K. Quite robust, ~1500 hp even without overboost, which apparently they did for more power (up to ~1700 hp) down at sea level, according to the American wartime memo on the British use of the Mustangs.
The P-40Ks don't hit the production line until May of 1942.
The P-51's were 55 aircraft held back from the British order for 150 planes (Mustang IAs) placed on July 7th 1941, First plane from this order was delivered in July of 1942.
The A-36 hits the production line in Oct 1942 after being ordered on April 16th, 1942.
The P-51A's were ordered on June 23rd 1942 to the tune of 1200 planes. Contract was cut back to 310 planes in Dec with the remainder to be built at P-51Bs.
British got not quite the number of Mustang IIs (it is a bit higher or lower) out of the P-51A batch that the US swiped from the Mustang IA batch.
The P-51A/Mustang IIs got the same engines as the P-40M/Ns of the time.
Now we have a discrepancy between the memo and Allison factory figures for power, compounded by the USAAF figures.
The 1500hp figure is overboost, no matter who's figures you are looking at. The question is how much overboost was allowed when and by who.
Once you are using more than about 44.6in of MAP the Allison was being over boosted. It may have stood up to it very well and Allison may have been lobbing for higher boost pressures but that is what the USAAF approved for "military power" and anything that exceeded that was overboost.
The exception were for the P-38s where the F & Gs were allowed 1325hp at 47in "military" and the H & Js were allowed 1425hp at 54in. "military".
Of course the P-38 engines used lower engine supercharger gears which makes up some of the difference. (engine can put more power into the prop because it is using less to drive the gear driven supercharger).
Maybe a few British Mustangs got -73 engines, strange things happened in the war, but most of the Mustangs Is had come off the lines before the -73s showed up and the -73s were ending their production run before the Mustang II/P-51A started their run. Allison having switched over to the -81 engines. Putting US ordered -73 engines into British ordered Mustang IA airframes in the spring/early summer of 1942 would have been a bit strange.
British only got about 21 of the P-40Ks unless there were in theater transfers.
British got 595 out of 600 P-40Ms but they had -81 engines.
Now for "official" over boost it seems that the -39 engines were allowed 56in at some point,1490hp at 4300ft (US didn't allow over boosting until Oct 1942)
The -73 engines were allowed 60in (1580hp at 2500ft), the -87 engines in the A-36 were allowed 52in ( 1500hp at 5400ft?) and the -81 engines were allowed 57in. (1410hp at 9,500ft)
What some squadrons did may be another story but going over those limits meant the pilots were acting as test pilots as far as the USAAC was concerned.
What the Allison rep said may have been different but what the Allison rep said may not have been a good defense at a court marshal for destroying US government property (engine/plane) in defiance of USAAC instructions/directions.
WEP is over boost.