I don't know the answer to this, however there are several reasons to think the convex side of a meniscus close-up lens should face the subject and the concave side should face the camera lens.
- Every screw-in close-up filter I've ever seen faces convex forward.
- When meniscus lenses are used as single element lenses for distant subjects (like in some older and Brownie type cameras), the convex side faces the film and the concave side faces the subject. That's the opposite of here, but the convex side is facing the closer optical conjugate (the film), similar to the way the close-up lens has the convex side towards the closer subject (de facto it produces an image at a distance of say 1m-infinity, which you focus on with the camera lens).
- There is a similar consideration, again with meniscus lenses, where the concave side faces whatever is forming the aperture stop. This is known to reduce aberrations, it is a worked-out example in a number of optics textbooks.
Anyway, my guess is with the Ambico system it will also be physically easier to insert the filter in the holder if you have convex side facing out.
