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Alex, The RKE's were designed originally for the Edmund Astroscan which was f/4 or 4.5. I have never seen a test indicating whether they include some compensation for field curvature and coma, but from the way they work in fast Newts my suspicion is that the 21 and 28 must have been designed with a curved field in mind and quite possibly a degree of coma correction.
It makes some sense to do that in telescope eyepieces because in the era they were designed, refractors and cats were very much slower (f10 - f/15) and the only scopes faster than f/7 were Newtonians.
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