An informal test of my Wollensak 18" f5 (approx) reveals the first principal plane is about coincident with the front surface of the middle concave element. The lens swung about a vertical axis in this plane shows negligible image shift for small angles of swing. Cooke triplet lenses, in my experience, tend not to be very fussy about whether the aperture goes in front of or behind the middle element. Even more convenient is to use a series of "lenscaps" with different size holes according to the apertures required. I can't find any image quality compromises with this approach and as expected image quality improves at small apertures.

But the reasons for me to shoot a 18" f5 aren't about small apertures so I always use it wide open.

In broad optical engineering terms the physically longer the lens and the more shallow the curves on the glass the better the image quality. The compromise then becomes reduced field coverage due to mechanical vignetting. Just look at all those long barreled Petzvals that only cover small plates. Incidentally when I measure just the glass, not the barrel, of my 18" Wolly front vertex to rear vertex I get about 103mm; a lens as long as it is wide!