I believe the Z53 is the new nib designed especially for the LAMY Aion, released about a year ago. From the product blurb on the Write Here web shop:
The LAMY Z53 nib is a slightly different shape from the other LAMY steel nibs, as it was designed specifically for the LAMY Aion fountain pen. It is still interchangeable with the Z50, but is rather more responsive than its predecessor. Much less pressure is required with the Z53, and it is much more expressive and responsive to added pressure.
The LAMY Z53 nibs are available in Extra fine, Fine, Medium and Broad, but note that Fine costs a little more, and Extra fine a little more again.
I don't know if I'd agree with the claims about it requiring less pressure. I wouldn't have said so, but now that I've read this I want to go back and put my Aions head to head with other LAMY nibs to see if I can see or feel the difference. The visual appearance of the nib is certainly different, though! Some might consider it a more feminine shape, but I quite like its elegant curves too. The product photo on the above-linked site is also especially illustrative of the difference (see below).
I have two of these on my own Aion Olive Silver and Aion Black. If a photo of the underside would be useful, I'm happy to provide one.
I personally find it very attractive on the Aion, but the design just looks wrong on every other LAMY pen I've tried it on. Likewise, when I've tried other LAMY nibs on my Aion (such as a 14k nib), it fits fine but it doesn't look quite right. What I've learned is that the standard LAMY nib shape is a design constraint that has prevented it exploring certain pen shapes, and that LAMY for the first time is experimenting with escaping that constraint (while still maintaining the trademark swappability we’ve come to expect).