I have a Stanley 71 and have fitted a mahogany base with inserts to bolt on and am generally very happy with it. However, despite having used the same measurements as Paul has recommended I find I am limited to about a 12mm cutting depth, ever with the knurled ring reversed. I’ve noticed Paul cutting at least as deep as 15mm with the same set up so are some of the aftermarket (Veritas?) irons slightly longer the the default Stanley offerings?
Perhaps your mahogany base is too thick? Remove it when you’re near depth or try a thinner material, such as Ply.
Vintage Stanley cutters were the same length as those by Record. Then Stanley changed to shorter ones (with the mushroom top). Paul might have been using a longer Record cutter in his Stanley router to reach the depth you see.