There was a real, from the ground up K model built. No photos are known to exist, and it was either destroyed, dismantled, or converted. The pictures are of the test mule (maybe as early as an E model to begin with) that was converted to match the K model.
The K had 1750HP (military power, 1875HP WEP) Allisons with fully counterweighted crankshafts and an improved intake manifold, similar to the G series Allisons.
The K had the three blade version of the Hamilton Standard High Activity Paddle propeller, it was around 13'6" I think. There was never a four blade version fitted.
The K required new nose cowls for the engine and propeller area, as the gearbox was different, as was the gear ratio, and the propellers were higher in relation to the engine. The War Production Board, citing the extremely high demand for the P-38, and the lack of second sourcing, denied Lockheed permission to shut down each of the two production lines for 1-2 weeks each, the time required to make the changes.
There was NEVER a Merlin powered P-38. Lockheed did a study and found the idea had no merit. The use of the Merlin in the P-38 would have added well over 1500 pounds to the air frame, and would have reduced both climbing performance and range. If you want to butcher a P-38, and make a pig out of it, hang Merlins on it. The P-38 had no need for Merlins, the turbocharged Allison F series V-1710 had all of the high altitude performance anyone could use, and maintained sea level HP far above any altitude any but the most special tuned Merlin could ever hope to.
The K model could climb to 20K from a standing start on the runway in around 4-5 minutes, it could reach over 450MPH in level flight, at altitudes high enough to reach compressibility in level flight, and had almost 20% more range than a P-38J. All of this was done with a rough hand fitted cowling, and in the case of the test mule, on an already well worn, used and abused air frame.
The Hamilton Standard propeller also reduced the load on the electrical system, since it was hydrostatic, as opposed to the junky electric Curtiss propeller. The single biggest flaw from the P-38J on was the Curtiss electric propeller. It wasted a good 20% or more of the power produced by the Allison engine, and taxed the electrical system. Had they just changed the propeller on the P-38, from the J model on, from the Curtiss to the Hamilton Standard, first to the three blade, and later to the four blade, the P-38 would have gained considerably in top speed, climb rate, range, and acceleration. Consider that even with the Curtiss propeller, the P-38 accelerated faster from cruise speed to 400MPH than any other fighter in the U.S. inventory until maybe the F8F.
