I have a standard Aerometric Duofold which pre-dates the move to numbered nibs and is stamped "PARKER DUOFOLD 14k PEN N" . I expect, but can't be certain, that this would be the same size as the later size 25 Standard nib. My Aerometric Senior, stamped 35, is considerably larger and much wider at the section (4.8mm vs 5.8mm). I can't imagine them being interchangable.
Not sure, but I raided my spares draw and this might help:
The top three are a 35, 25, and 10
The bottom 4 are the sum total of the loose "Duofold N" nibs in my possession. Whilst not all exactly the same size, I reckon most are nearer to 35 than 25.