what generation is iPad pro 12.9" model A1584 is

A1584 model is a 1st gen iPadPro 12.9".

These are the suggested keyboards:--> https://www.apple.com/shop/ipad/ipad-accessories/keyboards?page=1&f=ipadpro_12_9&fh=35d5%2B470e

Regards

Giulio

The link in my previous reply shows Apple suggested keyboards for a 12.9" iPadPro 1st generation, as per code A1584 you posted.

And these are its technical specs:--> https://support.apple.com/kb/SP723?locale=en_US

Regards

Giulio

The 3 keyboards in the link provided by Giulio above will work with a 12.9" first generation iPad.

For the first 2 length is irrelevant since they are not physically attached to the iPad.

The last one is specifically for the 12.9" model, not the 11" model. The only limitation is its only available in Portuguese, Russian and Swiss languages. No English version

Smart Keyboard for 12.9‑inch iPad Pro - Portuguese - Apple

More options here: https://www.amazon.com/s?k=12.9%22+iPAd+firts+gen+keyboard&ref=nb_sb_noss

The first two are not phisically connected to the iPad, the third is connected via Smart Connector and there is both the model for 11" and the model for 12.9".

Regards

Giulio