Even though he just joined the company and still has his L-plates on, he learns very quickly.

L-plates 是英国特有的说法，原指"Learner plates"（学习驾驶者车牌），是新手司机必须挂在车上的标识牌。这里用作比喻，表示"职场新人"或"初学者"的状态。L代表learner（学习者），plates指车牌，组合起来比喻新手身份的标志。