I can not tell if this gauge has ever been installed or used. While there are no obvious signs of use or having been installed, it may have been used at some point, I can't really say one way or the other. As you can see, it comes with the mounting bracket.
It comes in a Stewart Warner box, which DOES NOT appear to be the original box that came with this gauge. It also comes with a lamp holder, which may not be the one that goes to this gauge.No tubing is included, but Stewart Warner still sells the installation / tubing kit. From what I can see, this really looks like the same as the "Vacuum Inches" gauge, with "Manifold Pressure" face plate. You could probably use this as you would a standard vacuum gauge, but that is just my guess. The item "Vintage 1960's Stewart Warner green line Manifold Pressure Gauge" is in sale since Saturday, March 27, 2021.
This item is in the category "eBay Motors\Parts & Accessories\Vintage Car & Truck Parts\Gauges". The seller is "steve-n-tina" and is located in Garland, Texas. This item can be shipped to United States.