Looks like carbon steel for me, painted black to match the cross member. I have the brackets the same size as the footprint of the mounting clamps.

Here's an observation on the Accusump. It should be charged with 7psi. The guage that comes on it is 0-160psi range with 10psi increments. Try reading that. Why 160 pounds?Geeze.

Have replaced it with a 0-100psi liquid filled 2.5" max pressure indicating guage, with 1 psi increments. One will have to use a tire pressure guage to be accurate on the pre-charge pressure.

Zderf