Is there a difference between PC4-21333 and PC4-21000 RAM for iMac 5K 27' 2019
Crucial.com shows it being PC4-21300 so I would ignore Amazon.

See https://www.crucial.com/usa/en/compatible-upgrade-for/Apple/imac-%28retina-5k%2C-27-inch-3*0ghz-intel-core-i5%2C-2019%29#MEMORYFilters