Anyone know what this bracket is?

Yep, looks like the lower bracket for a Federal (Chrysler) power steering bracket for a small block engine w/ aluminum water pump. Indeed, from the same Bouchillon Performance bracket set I bought for my 1965 273 (after installing alum w.p., finding TRW pump wouldn't work, yada ...). That welded nut broke free on mine when installing. Insure that little bracket isn't missing if you buy a "Federal pump w/ SB brackets", since many pullers leave it on the block. Many post here w/ "WTF am I missing?", so if you want to sell, somebody will need it.