1957 introduced an enlarged cabin and the more powerful AI-26VF engine, which allowed the accommodation of three passengers on a bench behind the pilot. Cabin height increased from 1.22 to 1.26 m and width from 1.01 to 1.2 m. A noticeable difference was horizontal bottom windows line instead of slanted, with bigger rear side windows, and a less pointed fuselage nose. It could also be fitted with two external side capsules for the injured or mail. There were trials of an armed anti-tank variant Mi-1MU carried in 1961, being the first Soviet attack helicopter, but it did not enter production due to having a small payload and the cessation of production of the basic variant. Well over 1,000 of all variants were built in the USSR, including a proportion of dual-control trainers (with U suffix): Mi-1U, TU, AU, MU, with the instructor seated behind a trainee. 15 were produced in Moscow in 1950, 30 in Kazan in 1952–1953, 597 in Orenburg in 1954–1958 and 370 in Rostov in 1956–1960. In 1956, license-production of the four-seat model began in Poland, at WSK PZL-Świdnik, where 1,594 were built until 1965, under the designation SM-1. Several new versions were developed at Świdnik, including the SM-2 five-seater, with a new fuselage. Several international records in its class were broken with the Mi-1 or SM-2. Variants GM-1 The original designation of the Mil Mi-1 prototypes, powered by a AI-26GR radial engine (later AI-26GRF). Mi-1 Three-seat light general-purpose helicopter, seating a pilot and two passengers, powered by a AI-26GRF radial piston engine. Initial production model. Mi-1T Three-seat general-purpose transport helicopter, seating a pilot and two passengers, powered by a AI-26V radial piston engine. Mi-1KR (TKR) Light reconnaissance and liaison helicopter of 1956, basing on Mi-1T. Mi-1NKh Three-seat general-purpose utility helicopter, basing on Mi-1T. Designed to be used as an agricultural aircraft, air ambulance, passenger transport, air mail, freight transport helicopter (NKh—narodnoye khozyastvo—National Economy). Mi-1A Three-seat general-purpose transport helicopter of 1957, seating a pilot and two passengers, with increased reliability. Mi-1AKR Light reconnaissance and liaison helicopter, basing on Mi-1A. Mi-1U/TU/AU/MU Dual-control training helicopter variants (respectively, of basic Mi-1, T, A and M models). Mi-1M Four-seat light general-purpose helicopter of 1957, seating a pilot and three passengers. Visible changes are: raised roof above a cabin, blunter nose shape, and horizontal bottom windows' line instead of slanted. Mi-1M Moskvich Civil transport helicopter for Aeroflot, with better cabin soundproofing, equipped