RX5121 Wayfarer eyeglasses use the same longer 150mm acetate arm as Ray-Ban's sunglass Wayfarers in this size range, matched here for the square front's hinge position. This aftermarket pair is cut to that 150mm, acetate-arm spec rather than the shorter 145mm figure used on some other Wayfarer-family frames. Measuring the arm you're replacing takes a moment and avoids an easy mix-up between the two lengths.

The main wear point on an acetate Wayfarer arm is the embedded metal hinge barrel at the tip, which loosens gradually with use rather than failing all at once — you'll usually notice the temple starting to sit a bit looser before it stops holding altogether. Minor colour variation between a sun-faded arm and a new one is normal for acetate and not a sign of a mismatch. Replacing the arm resolves the loose hinge without needing to touch the front.