The Model M15 is the only mechanical buckling-spring keyboard made by IBM that is fully adjustable in terms of ergonomic position.
According to GreenPylons, it's worth about $1K these days. Back in the early 2010s (before the ErgoDox and other split customs) they would go for $1.5K as it was one of the very few ergo mechanical keyboards available at the time.
Highlights: Split Spacebar (obviously), two arrow clusters, and numlock indicator LED without having a numlock key or numpad at all (solution: it came with an external numpad).
President Bob Guenser sums up the history of Signature Plastics, creator and manufacturer of keycap profiles like SA and DSA, on occasion of preparing for retirement and searching for buyer candidates.
