I don't know about introduction of slushers, compressed air winches were certainly in use in Cornwall by the 1890s. The miners of the day would have been familiar with them from working in South Africa. They weren't much used in Cornwall as they were not usefull in the near-vertical stoping systems there; the Holmans one was a demonstration piece.
however they were used in coal mines at quite an early stage, before the introduction of the panzer conveyor and continuous cutter
The eimco rocker shovel was first introduced in about 1938 so I would expect that they first appeared in Cornwall somewhere after WW2
''the stopes soared beyond the range of our caplamps' - David Bick...... How times change .... oh, I don't know, I've still got a lamp like that.