The first twenty nine production P-38s were given no series letter, and are so normally referred to the P-38-LOs, to make it clear that it was these twenty nine aircraft being discussed, and not the P-38 in general.These twenty nine aircraft came from the first order for sixty six P-38s placed in September 1939. The thirtieth aircraft from that order was completed as the sole XP-38A, and the remaining thirty six as P-38Ds.The P-38-LOs were not combat ready. Most of them lacked guns, while none were equipped with self sealing fuel tanks. Delivers began in June 1941, and most were given to the 1st Pursuit Group, then based at Selfridge Field, Michigan. This was the unit that had carried out the service evaluation of the YP-38.The P-38-LO differed from the YP-38 in several minor ways. The original armament of one 37mm cannon, two .50 inch machine guns and two .30 inch machine guns had been improved by replacing the two .30 inch guns with two more .50 inch guns. Armour plating had been provided around the cockpit, and a sheet of armour glass had been placed inside the windscreen. During 1942 these aircraft were redesignated at RP-38s, indicating that they had been restricted to limited non-combat duties. (internet source) Enjoy it guys!