When we ran a full service chassis fab/turn-key hot rod shop a number of years ago, it was always useful to have threaded things around to weld into the frame for some kind of mount, etc.
Here’s a list of all our threaded things for your reference:
Shock Mount Frame Plate
Product Details
Brand: Welder Series
MPN: 103011
Country/Region of Manufacture: Canada
Thickness: 3/16"
The center of the notch would be 3" above the frame and 2-3/4" out from the frame. Usually used in pairs with a 3/4" diameter tube (see #10304) welded in the notch.