I suspect they come from the same hoard, were cleaned by the same process and then stored under the same or similar conditions. Chemical cleaning leaves reactive surfaces. Sulfur pollution in the air can produce yellow to brown to black toning over time. Likewise, materials used to construct coin cabinets impart various 'cabinet tones'. Toning tends to be more pronounced (sometimes iridescent) in protected areas which retain some degree of original mint luster, which appears to be the case here. Trace elements in the silver alloy may also be a factor. Lovely coins!
Edited by Kushanshah
11/23/2020 11:47 am