The Mandalorian is no stranger to Star Wars Easter eggs and the season 2 finale had one to C-3PO's silver leg. C-3PO has appeared in practically every major Star Wars production, and although he looks roughly the same in each film, sometimes he'll be wearing an alternative body part because something happened to the original one. This was specifically called out in his introduction in Star Wars: The Force Awakens, when he told Han Solo that he had a red arm, but this actually originated in the original trilogy.
It's difficult to see in the opening act of A New Hope since it takes place on Tatooine, but it becomes more noticeable later in the film and in later movies that C-3PO's right leg is silver. It's present all throughout the original Star Wars trilogy, though an exact reason for why C-3PO's leg is silver has never been given. Anthony Daniels told Wired in 2015 that C-3PO's silver leg was meant to represent his adventurous past, one that might've been filled with close encounters that resulted in him losing his original gold leg, which he did have in the prequels.
While The Force Awakens joked about C-3PO's mismatched color plating overtly, The Mandalorian gave Anakin Skywalker's protocol droid a more subtle nod. In The Mandalorian season 2 finale, Din Djarin and his companions board Moff Gideon's Imperial light cruiser to rescue Grogu; while Mando is making his way to Grogu's cell, he passes by a silver RA-7 protocol droid with a gold left leg - the exact opposite of C-3PO. It's too perfect of an Easter egg to be a mistake, especially considering how careful The Mandalorian's writers, producers, and production teams have been at referencing Star Wars' past, particularly things associated with the original trilogy.
The Mandalorian's C-3PO Easter egg is even better knowing that the droid used in the series is an RA-7 model. They first appeared in A New Hope and were identified as Death Star droids, since one appeared on the battle station and then later reappeared as inventory droids on Imperial Star Destroyers. Star Wars concept artist Ralph McQuarrie co-designed the RA-7 droids, which had similar bodies to the 3PO models; in many ways, they were identical in design and purpose, with the exception of RA-7 droids having insectoid shaped heads. So they could easily use and swap parts.
While the focus of The Mandalorian season 2 finale was on Din Djarin, Grogu, and Luke Skywalker, one of the best things about the series is that it has several deep cut references like C-3PO's silver leg which don't feel too over to be eye-rolling. They're subtle enough to come off as organic, which make them even more fun to spot.