I scanned the M15 diagram and I’m not seeing one spring right off hand either — I might just be missing it… but without tearing apart my M19, there are two springs that immediately see (videos attached… hopefully they'll go through). One I point out with a pen and the other — again, not immediately obvious in the diagram that I looked at, is in the left-center frame-portion of the vertical video. For some reason I remember that the one that went missing on mine was next to the platen, but I could be wrong. A screenshot of that portion next to the patten is also attached.