Birdseye49 Beautiful work ! I enjoy reading about your planes. However, I am not a hand tool user and do not understand the sprung versus unsprung designation.
Birdseye49.... here's an explanation...
Purpose: Molding planes are designed to cut specific profiles into wood. The "spring line" helps the user to hold the plane at the precise angle required for that profile to be formed correctly. In addition it helps to shape the profile cutting little at a time with each pass. Expecting a complicated blade curve to cut all in one pass would near be a blunt force nightmare
Location: The spring line is typically found on the front (or toe) of the plane, near the cutting edge.
How it works: By aligning the spring line with the edge of the workpiece, the user can be sure they are holding the plane at the correct angle to produce the desired profile.
Importance: If the plane is not held at the correct angle, the resulting moulding will be distorted or inaccurate. The spring line ensures that the profile is consistent along the length of the workpiece.
Not on all planes: Some molding planes, like hollows and rounds, don't have spring lines because they are designed for more flexible use and can be held at various angles to create custom profiles
