What I'm finding confusing is in true sizing, 28 bore is actually .55 calibre, while 24 bore is .578/8.
Is this gun being called a 28 bore becasue it's using 28 bore brass, even though it is a 24 bore by calibre?

The 4 bullet designs on the left were not even remotely duplicated (flat based) in the US for sporting use until the very late 60's early 70's seems to me - and minimally then as they were quite new for military breechloading rifles presented for testing in the 60's.