It is a little hard to tell from that picture, but I think that is an NA001459 header tank. Replacements are available, I think made by Seakamp:
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https://www.mrcool.us/na001459-perki...inum-tank.html
On the ones I've seen, which have been newer, that exterior feature has either been entirely missing or a vestigial indented ring.
Based on that, I assume it was a plug.
Because I think those plugs must have been an early feature, I'm not sure you'll find a replacement. You may need to improvise or, depending upon the overall condition, replace the tank.