PC4-21333 vs PC4-21300 RAM

They are one and the same.

Providing you can identify the build model year details specifically,

this should be easily found at least online. Then use info anywhere

that has correct matching specifications. OWC has exacting specs &

guarantees specified RAM modules will work; just get correct ones.

The iMac 27-inch model had two 2019 models; one batch, 4K.

And another one shows 5K. So there are other identifiers, too,

In a generalized fashion, an Upgrades for Mac (OWC) shows

both. [This page address has upgrades/imac19-1 for 27-in. 5K]

Your model 27-inch 5K iMac (2019) can use up to 128GB (4 pcs)

according details shown here: [128GB Memory Upgrade]

Examples each: All RAM Memory Upgrades, this Mac, here.

Good luck & happy computing!🌻☘

Both OWC and Crucial sell compatible PC4-21300 RAM modules.

Macs require exact spec ram, failure to do so will result in problems. Only buy quality ram from either OWC https://eshop.macsales.com/ or Crucial https://www.crucial.com/

Avoid other bargain basement brands.