Creating both long and short actions from multiple actions is still a requirement for gaining membership here in the U.S. into the American Custom Gunmakers Guild. Both have been popular in the past as a demonstration of your metalworking skills and knowledge of firearms functions. The long actions generally went towards the cartridges used for hunting the Big Five while the short (Kurz

) was used for the wildly popular (at the time) .22-250 or other cartridges of that same length. I don't remember any secretive procedures since the process is straight forward as long as you measure 5 times and cut only once...
As to the upper echelon of Mauser actions, Reimer Johannsen and Gottfried Prechtl still make what are acknowledged as the best M98 pattern actions. Modern machining technology is applied but the same pattern actions are the result. The prices reflect the technology as well as the handwork necessary to provide the very best actions, usually running $3,500 per action and up based on accessories or modifications requested. This does not include engraving which is another totally high end indulgence. The Satterlee actions are an American entry into custom manufacturing. Granite Mountain actions, while expensive, often require addition fitting to become the exceptional actions for custom builds. The draw though is the actions sizes and magazines are based on the bigger, longer cartridges so popular on Safari for dangerous game.
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.500 (12.5x70) Schuler (.500 Jeffery) in a Prechtl actioned rifle: