I was under the impression that this Cessna C210 was a turbine engine modified aircraft with a Allison B17B engine or similar model engine in which case full feathering is not normally available in flight, only upon engine shutdown. (oil pressure)
Perhaps it would be the same engine STC as the A36 Bonanza modification.
Johnson & Johnson in the USA operate or at least used to, a Nomad on floats at the bosses lake house. This aircraft had modified engines so that full feathering was available with engines running on the ground/water.