Q: In the visibility section, it says ‘units in buildings and entrenchments or under concealment may be seen at 2-40 inches. Is that correct? Do you mean 2-4 inches?
A: No, it's as written. Patrols poking around at 2 inches, or officers with good field glasses at 40 inches both have a straight 40% chance of noticing "hidden" units.
Q: On page 3 of the 1870 rules it says that units beyond 40 paces should be described to the player in the suitably vague terms of ‘small’, medium’ or ‘large’. Can you give an example of what constitutes a ‘small’, ‘medium’ or ‘large’ force? i.e. regiment = small, brigade = medium etc.
A: Kind of depends on the size of the scenario… if there’s several corps involved on the board, a “large unit” could be a division-plus in size; if only a few divisions, then a correspondingly smaller unit might be described as “large” …perhaps a brigade or more.
Q: What are the benefits of hedges and stone walls in the rules regards cover from fire. We assume that hedges offer concealment (as per visibility rules) but no fire defense advantage however stone walls offer both concealment and some protection…
A: I suppose that some very dense, high hedges would adequately block line-of-sight and conceal whole units, but that's very much case dependent. Such hedges existed in Italy, and in Norman bocage territory... but a blanket rule that hedges conceal units might be subject to a lot of abuse. A nice little garden hedge concealing a whole battalion of infantry? Don’t think so! Generally, if I came across accounts of hedges being extensive enough to do that, I'd put it in the scenario notes as a special case (example: Magenta or Jagel). I can't think of any FPW battle where hedges were tactically significant. I allow stone walls, railroad embankments, and deep ditches to provide cover and concealment to units in extended formation only, and in 1864 Denmark, the well-banked knicks (hedge-topped dikes) get “field works” protection from small arms (and sometimes artillery) fire, besides counting as linear obstacles to movement – just like stone walls.