From the Greenberg Flyer Wide Gauge book, The earliest Flyer freight cars were Lionel 1x cars with Flyer stamped on the bottom and had the black flex truck starting in 1925, in 1927, they came out with their own designs but continued with black trucks. The gray trucks started about 1928. Being as Flyer left the Lionel graphics on the early cars, a quick truck swap on a Lionel 14, 13, 12 or 17 would give you a decent fake early car.
Steve