It carries a little label which explains the machine, but not its storage location:
And here’s one in operation (a platen press, not a men’s room). The video is less than a minute and half long, so just click that arrow. If you are getting this via e-mail the YouTube video may not show, so click on the title at the top of the post to view it in your browser.
A nice symmetry can be seen in the background, behind the operator’s head.
The disk shaped part is not the platen. In the video you can see the ink rollers running over the disk in order to spread the ink evenly across the image area (this is the function of the disk) before running below to ink the type. The platen is the plate onto which the operator is putting the paper, and when it moves down it presses the paper firmly against the inked type.
More history can be found at Letterpress Commons.