Many techs find that using the sostenuto to hold up dampers of keys being tuned, allows for better hearing of the partials.
This photo shows how you can use mutes to hold up the dampers on pianos with no sostenuto.
September 2
Repairing, Regulating, and Evaluating Pianos
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This photo shows how you can use mutes to hold up the dampers on pianos with no sostenuto.