I've seen Macs be REAL finicky about RAM before. And if they say PC4-21300, then PC4-21333 likely isn't going to work. It's their way of making you spend more by buying it direct from Apple, or AASPs.

It's possible I've never dealt with it, just laying out my opinion on it.
If they've gone as far as making RAM exclusive then thats some devils work right there.

Only thing I've dealt with for ram was needing to buy there OS upgrade way back to allow the shit box to take more ram.
(mind you this was moving from 2gb to 4gb and according to the hardware specs it supported 8gb but OS limited to either 2 or 4)

Apple did the same thing in earlier generations with PC3-10600 vs the more common PC3-10666 back around 2010-2011 (My wife still has a 2011 iMac that I did a RAM upgrade on back in the day...I had to get the less common - and more expensive - PC3-10600 RAM for it).

I really hope not.



I looked for a few PC4 21333 RAMs on Amazon, some are 260pin.

Now my only problem is I purchased a PC4 - 21300 260 pin DDR4 SDRAM and Apple says to use PC4 - 21333 260 pin DDR4 SDRAM
So I am not sure if they would be compatible

you won't damage it trying it so worst case scenario you bought wrong set and need the right one.

PC3-10666 was not compatible with that iMac where PC3-10600 was recommended?

What would happen if I insert a non compatible one? Does it cause issues or just not be detected?

it won't post.
just sit at a black screen.

cool. I hope it works :/

do post back and update us on results