Thanks for the replies, folks, and very sorry for my late reply on this... I've been busy collecting Rolemaster and Shadow World stuff.
I still don't know the answer to this question. I've picked up several copies of the SW Master Atlas 2E and only one of them has a little bump between the pages where a map could possibly have been inserted. The funny thing is though, that copy and its map came out of a box set that was called Shadow World Supreme. See photos attached.
Shadow World Supreme, in a rather dodgy way, uses the SW Master Atlas 1E box but has a sticker on the front saying: "Shadow World Supreme - Contains the following: Shadow World Master Atlas 2nd Edition, Sky Giants, Cyclops Vale, Nomads of the Nine Nations, Norek. Total Retail Value: $66.00. Suggested Retail Price: $40.00"
A "Shadow World Supreme" seems to suggest to me that there was a non-Supreme version. Could ICE have sold the SW Master Atlas 2E and its map in the old 1E boxed set? And that's how it got around the issue of the map being oversize?