... sometimes called a support flange or transfer bead (on a glass bottle it's a collar).

This is an example of a bottle preform and another example that is specifically used to make a milk bottle.
https://www.esterform.com/theme-content/uploads/2016/10/E823...
https://www.bmimachines.com/wp-content/uploads/2015/09/512px...

You can see on this link (page 7 of 79) the 'before and after' showing the preform before moulding and the bottle that is produced from it after moulding
https://insight.rpxcorp.com/litigation_documents/14042791

This dictionary gives aleta as one of the translations of flange
https://books.google.fr/books?id=ymGOeyga09AC&printsec=front... (page 189)

Other translations of 'aleta' do include eg flap, fin, wing, paddle, flipper, but as you can see there are none of these present on a PET bottle preform.

The only places I can find 'aleta' (not referring to a flap or fish fin!) used on anything is where it is a rim/flange e.g.
https://norces.com/?s=aleta&post_type=product
https://mx.etwinternational.com/square-tube-with-flange-5879...
https://carbonestore.com/products/trc-41-perfil-marco-fijo-7...

...or where it is used to describe cooling fins on a pipe e.g. here
https://itisamexico.com.mx/tubos-aletados/

In English, the parts of a preform are shown at the bottom of page 7 of this link https://insight.rpxcorp.com/litigation_documents/14042791 which does look a bit like the fin on a pipe!

Other bottle anatomies here
https://www.qorpak.com/pages/anatomyofabottle
https://www.oberk.com/packaging-crash-course/bottles-101-the...