Curious, I really don't know. But when these plastic cars, etc. are molded. Aren't dies made to make multiple smaller parts, compared to large parts which maybe only one to a mold?
Working for the Chemical Industries, I've visited many plastic manufacturers. One I remember visiting was Revell Plastic. The molded numerous extra small sprue, that were not needed but were replacements for broken or missing on the assemble boxing line, and also catalogued in case of customer not treceiving, missing or broken sprue pieces.
When I had a missing horn on an Atlas Switcher, the nice lady told me thay molded extras and would send me 2, also what color I needed. Woodland, had broken lamps on a building. They said to return it, or if I could repair it, they would send replacement parts. MTH, had broken pilot, trailing wheel axle bent, coupler broken, cracked PS3 board, etc. replacement sent.
Lionel, broken coupler, bent axle, missing bell. Still waiting for replacement parts
