It is called an "axle slider". Maybe someone can correct me if wrong,
but all it looks like is a section of angle iron with spring hangers
welded on it. This seems like it would be handy when building a
trailer to "slide the assemply with the frame upside down to the exact
spot where the centerline of the axle is the same distance from the
tongue on both sides, then weld the angle.

I am just throwing out questions trying to learn. I may or may not
build this thing anytime soon but I wonder since this is a critical
weld area if I could get a pieve of angle, say 2x2x3/16, have an
experiecned welder weld the bracks to the angle after I have tacked
them in the proper position, then, once the "sliders" are in the right
place, drilling and taping theangle through the bottom part of the
frame tubing, bolting it, then also welding around the angle?

Just thinking out loud here. Is there any advantage to using one of
these "sliders"?