Windows 10 build 14393.51 is the latest version of Windows 10 Anniversary Edition, which was a feature upgrade. Many new and exciting features in this version.
To be more precise, yes 14393.51 is a cumulative update to version 1607, which was the Anniversary Edition. On Tuesday of last week, Aug. 2nd 2016, Microsoft released 1607 build 14393.1, which was very quickly replaced by 14393.5 which was quickly updated to 14393.51.

It was an interesting week for us Windows 10 insiders.