A three digit stamps was used on doors and opening latches on cars up to the 22xx cars. After that four digits were used.
Notice the support strap on the center ferrule for the side curtains? 427's lack that support strap. You can see this if you search photos of csx3042 and others.
If you look at your photo near the inside latch, you'll notice what appears to be blue paint which is the original color of 2124.
While it's possible that "left over" leaf spring parts were used on early 427, I still think it's a leaf spring door