Phillip23

There is also the option of having the Century WA adapter become the variable diopter using one of Pooli's helicoids.

This eliminates the need to double up on glass.

The weak link is the Century itself. The quality of the image towards the edges gets not-so-good.

On the plus side, it seems to help with flattening out the Kowa's uneven side crunch and gives shockingly close focus.

Can someone tell me what the advantage would be of the full Rectilux 3FF over this core DNA?

One advantage on the Rangefinder would be focus marks, but I'd choose non rotation of glass to be more important.