P90's over the years have been made with: Alnico V and Alnico II magnets. And coils from about 7.5k to well over 10k.. 12.5k...
Gibson has changed the recipe many times. I suspect they have even changed it based on the model of guitar it was meant to be in.
It has been established that they do this currently - whereas - some production is potted, some not, some has enamel coated wire some has other coatings... all the same pickup "model name" but made very differently depending on its application.
A P90 must be as made by Gibson, or a clone of it in some form.
When you start talking about P90-type pickups, then what you're referring to is a single coil pickup, whose magnet and/or magnets are under the coil, with steel poles that are(or are not) adjustable. A Strat type pickup has magnetic poles, and no under coil magnet is used.
Putting the magnet under the coil allows for greater magnetic pull as larger magnet(s) are used. It also gives a wider magnetic field, especially if the design uses two magnets (as it should, but not all do).
A Fender Tex-Mex pickup is a "magnet under coil" pickup.
A single rail pickup, using a plain steel rail and two magnets below the pickup is a P90-type pickup.