That's known as plug welding, but I don't believe its weld you're seeing. The technology to do this, especially on such a small scale, didn't exist back then. Besides, the spots are too perfectly round for that anyway. The rings around the edges suggest to me they are holes that have possibly been tapped/filled with screws and made off flush.
If they was bucked good they wouldn't. Rivets bodies can be a bugger to drive out.