Windows 3.1 build 50 is referenced in a series of Microsoft e-mails from September 30th, 1991.
These e-mails mainly discuss application compatibility with Microsoft Windows 3.1 Beta 2, though this particular version is mentioned in relation with SmartDrive (codenamed Bambi) as "other critical work".
It was also mentioned in the same series of e-mails that the development team planning on putting code in place that would make sure that Microsoft Windows 3.1 would only run on Microsoft MS-DOS, since only Microsoft MS-DOS and IBM DOS were supported for running Windows.
