55.2-59 Ol' Blue

[dsraven]

those frame brackets are usually associated with overload springs. on this style there is a set of leafs added above the regular springs. they don't attach to spring hangers but simply sit there above the regular spring stack. when a load is applied heavy enough to lower the truck enough, that stack of overload leaves will bottom against the frame brackets and start to take some weight
these sites has a pic if you scroll down just a little
https://www.torklift.com/rv/stableload
https://mrtrailer.com/psrsuspension.htm