The RT11 V5.1 Distribution.

The three files in this directory contain distributions of the RT11 Version 5.1 Operating System.
RT51F is an archive of RX02 floppies.
RT51D is an RL01 drive archive.
RT51T is a MT0 tape archive.

There are additional floppies which contains utilities for the MTI controller, and the TSX11 Operating system.

Install images for TSX-Plus 5.1
http://www.rsts.org/autoindex.php?dir=distros/RT51/

It says RT-11 5.1 there, but one of the archives also contains TSX-Plus 5.1.
I have installed the TSX-Plus 5.1 images.