Stanley No. 5 has been made about 150 years now. Although on a quick look they may all look the same, there have been a wide variety of finishes and materials in use. And quality has not always been the same. I’m not deep in to the subject as I’m not a Stanley man myself, but as I have understood they can be dated quite accurately by studying the casting markings and so on. I would start there if I was going to restore the plane. If against all odds, it turned out to be a golden era specimen, I would think twice before proceeding with restoration. If it was from not-so-desirable era, I would just strip all the finish layers and re-coat with whatever clear coat I happened to have in hand. I think it would still make a decent jack plane for fast wood hogging.