Variant 3 is totally blank except for the map.

The reason the map positioning makes it likely to be a Clowes book rather than a Billing copy is that in the Clowes 1st Impression the map is on the left hand page while the Billings 1st Impression (and all later impressions) had the map bound in the other way round as the right hand page.

These three bibliography entries record the map facing an odd page number for the Dummy and Clowes 'Export' Editions, and facing an even page number for the Billing 'Domestic' Edition and later impressions:

Dummy
Export
Domestic

I guess the people buying the Folio copies aren't 'hardcore' collectors and just want a good looking set of books. The Folio editions fit in nicely (price-wise) between trade hardback editions and the various HC deluxe editions.