The EXC from B23.423 (B23.424 is for scaling) refers to mortars and rooftops (B23.85).
V7 refers to O.5.5 and O.5.4 but it is O.5.6 that allows every gun to set up in a RB factory.
On a recent VotG thread, Tom Morin writes he likes to place a Russian 82 MTR on a level two gutted building. If he says so it should be legal but as I read the RB I can't find something to back up such a placement.
I answered a bit too quickly...sorry. Reading further, section V7.2 references all of section O5.4, which as a subsection contains O5.45 which does allow mortar fire from a roofless location.
I do see what you mean about O5.6 not being mentioned....but the exception in B23.423 for mortars allows them to be place in an upper level location, although they can only be fired from a rooftop location in a building where a roof is present.
B23.423 GUNS: No weapon depicted on a ⅝" counter may occupy an upper level of a building [EXC: Mortars (23.85); Fortified Building (23.93)].
and
B23.85 GUNS: No ⅝" counter weapon may occupy a rooftop, except for a mortar of ≤ 82mm which must be dm to be portaged to/from the rooftop (or between different Location of it). Such mortars may fire from those rooftops. A Gun may never be Emplaced (C11.2) on a Rooftop.
An argument could be made that the exception in B23.423 which calls out B23.85 which in turn only allows mortars as the only 5/8" gun on rooftops....not sure I agree with that argument, but it can be made.
There is another exception to B23.423. Not sure if this is what Tom was refering to, but a 5/8" gun counter can be placed in an upper level if that upper level location is fortified which implies all the lower levels below it are fortified. This is listed along with mortars as an exception in B23.423. As many fortified building locations that VotG seems to grant, a very real option.
I've used it for AT guns to peek over roadblocks....
I seem to recall somewhere in there I did see words to the effect that AA fire was not possible from a roofless location....but I cannot find the specific citation right now.