Well, it seems I've answered a few of my questions:
1) This is not an "only-Nagashima" set, as I tracked down a few more (from searching through ebay listings of uncatalogued materials)
2) That this is indeed a 1958 set (since one of these new cards depicts Kaoru Sakurai, who only played for the Whales in 1958). So these 2 Nagashima cards will be added to my "rookie card" list.
I'll have to add this set to my list of uncatalogued cards on my blog...