I have very little personal experience, but have researched fairly thoroughly. I am in the process of translating an account of Erard action prep written in the late 1860s (presumably by an Erard factory manager). It was uncovered by Robert Adelson after he published his book,
Erard - A Passion for the Piano. It is included in Marc Valdeyron's translation of that book into French, which came out late last year. The following regulation specs are derived from that account.
Blow 54 mm + (adjusted by a linked "capstan," a reverse threaded brass rod, square in cross section in its middle, that turns in two wooden pieces, one attached to the key, the other to the wippen)
Let off 6 mm
Dip 8 - 9 mm
Check is not adjusted by bending the "spoon" (if it is adjusted, that is done by replacing the leather on the hammer molding that bears on the check), but should be about 25 mm
Drop (the screw is on the shank itself (not the flange), and is T-shaped - the T bears on the rep lever) can be adjusted by turning in 180 degree increments). 11 mm is recommended
The dampers are below the strings, lowered by a kind of spoon that extends from the end of the wippen.
The photo shows the parts. You can remove a key without pulling the stack, but must unsnap the equivalent of a captan from the key first.
Not sure if you will have the original flange design, which allows for adjusting friction by turning a screw.
That should at least provide an initial basis for understanding. It's a different animal.
Note that the hammer is mortised to the shank - far more stable than a dowel glued into a hole. Checking is barely felt due to lever advantage (compare to our familiar design where it is 1 : 1, check on end of key to finger on key). Damper action is also felt far less, again due to lever advantage. The linked "capstan" design is far more stable, doesn't need constant adjustment as the wipp cushion compresses. The jack tender is covered with felted cloth, which means that it will not create dimples in the let off cloth (again, more stable regulation). These are the kinds of things Sebastien Erard obsessed about.
It is a very sophisticated design, but difficult to manufacture. The modern grand action is a simplification, removing some of the most important details, from the perspective of the feel and facility of the action.