I think, in the original design, that 14mm was the longest focal length possible with the design, which is why there is a 14mm.
The 17.5mm was released over 3 years later, and went through at lest 3 prototypes before they got it right.
Even then, the final version has less apparent field than the others (72.2° versus 78.3-79°), a much longer eye relief,
and a visible difference in correction.
Once the 17.5mm came out, though, it was easy to see what Baader had in mind for the magnification progression.
Like the 1.25" Pentax XW, they follow a 40% change between focal lengths, but only if you exclude the 14mm.
Perhaps for this reason, the 14mm became the least popular focal length.
Ernest Maratovich's lab tests showed the 17.5mm to be superior to the 14mm, but I see the reverse.
I can't account for that. Is it individual eyepiece variation, or something about my eye at that exit pupil, or some other issue? I don't know.
I do have longer focal length eyepieces that are sharper than the 17.5mm, so I don't think it's the correction in my glasses.
Also, I have adjusted the eyeguard positions on all the Morpheus so they are identical to my glasses.
I inserted thin o-rings under the eyeguards on a few of them to get the eye relief to exactly the point where, when my glasses touch the tops all the way around the rubber, I *just* see the field stops.
This eliminates any personal feelings about eye relief in evaluating the individual focal lengths.
I recently did this after some other posters' comments, and it made a difference the first night I used these on the Moon.
As has been reported, if you hold your eye the wrong distance away from the eyepiece, there is some chromatic aberration of the exit pupil that yields a brownish tint to the very outer field.
I see this in the Pentax XWs when used in daylight, except there the coloration is yellow.
Using those thin o-rings has allowed me to optimize each focal length for my glasses, and now I do not experience the edge coloration when viewing the Moon.
I mention this because they are very capable Moon viewing eyepieces if you manage to optimize your eye position relative to the lens.
As they come out of the box, you have at least 4 different positions for the eyeguard, not to mention the eyeguard can be unscrewed a MM or two, resulting in a huge range of adjustment.
I think it's worth the experimentation, because it makes the focal lengths more uniform and comfortable to use.