Face Normal.
Have you looked at the following? This particular script requires TSE, but
all the info is there….
[74664,2144] Martin Doudoroff/KUB Lib:18
SELECT.ZIP
Bin, Bytes: 3467, Count: 187, 17-Mar-95
Title : Select Faces by Normal (Script'd utility)
Keywords: TSE SCRIPT PXP SELECT FACES NORMAL UTILITY 3DE
Fantastic script for The Script Extension PXP. This utility selects faces
on multiple objects according to the direction of their normals–something
you cannot do any other way. Say you want all the faces which are
oriented vertically? SELECT.P3D does it in a flash. Fully documented code
with a nifty user interface. Check it out. (Requires TSE.)
(DL time at 2400 baud: under 1:00)