and reduced control forces for the pilot. This production block and the following P-38L model are considered the definitive Lightnings, and Lockheed ramped up production, working with subcontractors across the country to produce hundreds of Lightnings each month. Two P-38Ks were developed from 1942 to 1943, one official and one an internal Lockheed experiment. The first was actually a battered RP-38E "piggyback" test mule previously used by Lockheed to test the P-38J chin intercooler installation, now fitted with paddle-bladed "high activity" Hamilton Standard Hydromatic propellers similar to those used on the P-47. The new propellers required spinners of greater diameter, and the mule's crude, hand-formed sheet steel cowlings were further stretched to blend the spinners into the nacelles. It retained its "piggyback" configuration that allowed an observer to ride behind the pilot. With Lockheed's AAF representative as a passenger and the maneuvering flap deployed to offset Army Hot Day conditions, the old "K-Mule" still climbed to . With a fresh coat of paint covering its crude, hand-formed steel cowlings, this RP-38E acts as stand-in for the "P-38K-1-LO" in the model's only picture. The 12th G model originally set aside as a P-38J prototype was redesignated P-38K-1-LO and fitted with the aforementioned paddle-blade propellers and new Allison V-1710-75/77 (F15R/L) powerplants rated at at War Emergency Power. These engines were geared 2.36 to 1, unlike the standard P-38 ratio of 2 to 1. The AAF took delivery in September 1943, at Eglin Field. In tests, the P-38K-1 achieved at military power and was predicted to exceed at War Emergency Power with a similar increase in load and range. The initial climb rate was /min and the ceiling was . It reached in five minutes flat; this with a coat of camouflage paint, which added weight and drag. Although it was judged superior in climb and speed to the latest and best fighters from all AAF manufacturers, the War Production Board refused to authorize P-38K production due to the two- to three-week interruption in production necessary to implement cowling modifications for the revised spinners and higher thrust line. Some had also doubted Allison's ability to deliver the F15 engine in quantity. As promising as it had looked, the P-38K project came to a halt. The P-38L was the most numerous variant of the Lightning, with 3,923 built, 113 by Consolidated-Vultee in their Nashville plant. It entered service with the USAAF in June 1944,