The authoritative source for Mach releases and updates is www.MachSupport.com
The Mach "Lock Down" releases are all available from the ArtSoft USA site and some of the Mach3 development releases are also available from their archives.
However, not all the development releases are hosted on the MachSupport site. ArtSoft USA has kindly given CVI permission to host for download key Mach3 releases for MSM customers.
Mach3 3.43.37- this release has proven to be a popular, stable release for use with MSM 1.x.
Mach3 3.43.38- this release is the minimum Mach version required for MSM Turn and Mill-Turn.
Mach3 3.43.66 - This Mach release fixes a few more Mach lathe related bugs and is required for some lathe features that were introduced in MSM 184.108.40.206 beta. Without at least this Mach version, the new features (e.g. Front/rear tool post editing) will not appear in MSM (See the MSM releae notes for more information). This is the version which CVI currently recommends for use with MachStdMill.