more info on Minecraft Forum
or on GitHUB
these files (with highest end number) will always contain the newest AdditionalBuildcraftObjects-code - so use at your own risk :)
the files numbers are named like this:
Z.Z.Z/buildcraft-Z-additional-buildcraft-objects-X.X.X.Y.jar
where
X.X.X is the (release) version
Y the build number
Z.Z.Z is the buildcraft version.
Most people want the latest version in "4.1.0" for Minecraft 1.6.4
Latest version for 1.5.2: buildcraft-Z-additional-buildcraft-objects-1.0.6.159.jar
Latest version for 1.5.1: buildcraft-Z-additional-buildcraft-objects-1.0.3.133.jar
Latest version for 1.4.7: buildcraft-Z-additional-buildcraft-objects-1.0.2.108.jar