Emacs-25.3_1 is a Windows only bug-fix release.

It is functionally identical to Emacs-25.3, except for a fix to the
Windows build process. The original 64 bit binaries for 25.3 were not
able to run on all target platforms. The 32 bit binaries are
unaffected.

The changes are available in commit 7b3d1c6beb54ef6c423a9, and the
issues fully described on the emacs-devel mailing list.

https://lists.gnu.org/archive/html/emacs-devel/2017-09/msg00925.html

Source code is available in emacs-25.3_1.tgz