Those hammers were probably pressed with a lot of steam, heat, and pressure. And, from your description, a hydraulic press.
The felt, which would have been too large for the hammers, would have been saturated with steam to make it flexible enough so it wouldn't tear when forced into the bottom caul. The press, being hydraulic, doesn't care how much pressure is being applied -- there is enough available to crush molding if the stop screws are not adjusted correctly.
The side cauls are also hydraulically operated but there were probably no stop screws controlling their movement. So the pressure on the sides from the parting line back would be pressed very hard.
There would have been a lot of heat used. This reduces the curing time of the glue and increases production. It can also burn the felt on the sides. The tell-tale brown crust on the surface can be cut and/or sanded away so the technician will rarely notice. Look for a very slightly tanish color to the felt from the shoulder back.
Assuming this is how the hammers were pressed -- and without looking at them personally I am only guessing -- about the only thing you can do is try to swell the felt up some. Conventional voicing won't do much until you turn them into marshmallows at which point they just go 'thud.'
Try dosing a couple of the hammers with a 70% solution of alcohol & water. Does it soak in? If it does saturate the hammers with this mixture and let it dry overnight. The hammers will swell, sometime a little, sometimes a lot. Sand them back to their original size and see what you have. You can do this more than once if necessary.
This is not a perfect solution to the problem -- that would be replacing them with real piano hammers -- but it can make their sound a lot less bad.
ddf
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Delwin D Fandrich
Fandrich Piano Company, Inc.
Piano Design and Manufacturing Consulting Services -- Worldwide
6939 Foothill Ct SW -- Olympia, WA 98512 -- USA
Phone 360.515.0119 -- Mobile 360.388.6525