Note there are at least two different sizes of 'flat' washer, and hence I guess spacer too.
FRC8002 has a slightly smaller hole than FTC3179. Obviously the radius is the same as the bearing is the same, so it is just the position of the flat. FRC8002 will not fit on a stub axle designed for FTC3179. FTC3179 will fit on a stub axle designed for FRC8002, but may not lock as effectivly due to small amount of rotational play.