Page 39 of Donald Dallas' book "Holland & Holland 'The Royal' Gunmakers" gives a picture of Holland's setup at their Kensal Range in the late 1900's, with a brief description of how their regulator, William Froome went about the business.
The stand that he is using appears to have a series of linkages that presumably allow for variations in the shooters height and/or stance. The main thing is to get yourself into a position that will allow you to move back with recoil, which is not possible when leaning forward in a sitting position.
I will e-mail some scans from the book for you to you to throw up, if that's not in breach of any copywright.