Buying the Correct Circuit Breaker?

How do I know which breaker will work for my house? It's from the early 70s. The main hot wire is loose, the screw to tighten it is striped, and the light keeps dimming. The only mark I can find is 2 little labels, one says: und lab. inc. list circuit breaker, 2 pole unit issue km-43, 120/240 v.a.c. ctl type qitf or q1tf, square d .co [60] and the second says: lug, cu 6-0, al 4-0.

By Andrea