Most American Flyer in the O and S gauge of that era use what are series wound motors - or universal motors. These were initially designed for use with the AC toy transformers of the day, but can be run on DC without major issues*. These items generally do not have a suffix, but occasionally an AC suffix will be applied.
As a presumed cost cutting measure, some American Flyer locomotives were manufactured with DC motors. These cannot run on AC. These almost always have a DC suffix.
Now, to further confuse things, more modern Flyer have "DC"/can/"maintenance free" motors, but have conditioning/command control circuitry designed to pull AC, not DC, from the rails. Obviously, this continues the roughly century tradition of low voltage AC on the rails. This command control circuity can be damaged by DC on the rails.
*These motors run hotter on DC and can result in unwanted magnetisim
