There are many such things but beats me how this is one of them. Brass is a common material for tacks, iron/steel for nuts and bolts. The reverse artifacts exist, but are specialized. Plus the repeating _a_s sound obtained by "brass tacks" and at two words rolls off the tongue (similarly with "nuts and bolts"); "brass nuts and bolts" would be a terrible phrase both because it refers to something uncommon, and because it doesn't roll of the tongue. Seems easy. C'mon!