I struggled with that too - but I think it's because the first (pre-/) line is the only one not to be lettered, in the diagram it's `1a` not because it's `a` on `1`, but because it's `a` on the `1`-th branch off `1`; in total it's `1/1a`.

So `3` is the third branch off `21`, `d` marking that it's the d-th (fourth) in line on that branch.

But maybe I'm still wrong, I agree it's not clear.

Edit: Having said that, even by what I'm saying it should end `53a`, not just `53`.

It’s a branch system. So that string means literally: Card 21 -> 3, d -> 26, g -> 53