150mm is a longer temple than Ray-Ban uses on most of its square-front acetate models, and it's the length built into the RB4340 Wayfarer — matched here in this compatible pair, along with the acetate construction. Because the extra length changes where the arm's balance point sits, a 145mm arm that fits a similar-looking Wayfarer won't sit the same way on the 4340. That's also why a 150mm arm salvaged from a different Wayfarer model, even an acetate one, isn't a safe substitute here.

Longer acetate arms flex more over their length than shorter ones, which is normal and doesn't mean the material is failing — the actual wear concentrates at the metal hinge barrel near the tip, same as on shorter acetate temples. If that barrel loosens or the screw won't hold, replacing the arm is more reliable than repeatedly re-tightening a worn thread.