This is what I have for my Passenger 738:
[General]
atc_type="BOEING"
atc_model="B737"
Category = "airplane"
icao_type_designator="B737"
icao_manufacturer="Boeing"
icao_model="B738"
icao_engine_type="Jet"
icao_WTC="M"
icao_engine_count=2
performance="Cruise Speed:\nMach 0.789 (455 kts\s 842 km\/h) \n\nEngines:\nCFM56-7B26 \n\nMaximum Range: \n2,935 nm (3,377 mi\s 5,436 km) \n\nService Ceiling: \n41, 000 ft (12,496 m) \n\nFuel Capacity:\n6,875 U.S. gal (26,022 L) \n\nEmpty Weight-Standard:\n91,300 lb (41,412 kg) \n\nMaximum Gross Weight:\n174,900 lb (79,333 kg) \n\nLength:\n129 ft 6 in (39.47 m) \n\nWingspan: \n112 ft, 7 in (34.32 m) \n\nHeight: \n41 ft 2 in (12.55 m) \n\n"
Think I found the issue: icao_type_designator="B737" needs to also be changed to icao_type_designator="B738"