I think the IBM M2 and N2 is flat.
Show Image
(http://i1191.photobucket.com/albums/z479/slaktarnp90/IBM%20model%20M%20och%20N2/MbakomN2.jpg)
This. (Though I have no clue what an N2 would be.)
I can't say a flat BS board would seem too sensible to me though. That's the kind of keyswitch that invites oldschool "pecking" style typing with a rather high hand position. Curvature is quite handy there, which is precisely why it was included in the first place.
I'd say get your sitting posture and keyboard height right and go with a regular curved M.