The M is standard as you note for the 2021 release. It is a polymer insert to to help improve launch on long irons. The 2019 version had a different mounting design. Looks like 2023 version (T350?) may not have the same exposed insert as the first 2 T series.
Check the 4-5-6 irons carefully. My 6-48 set is only on the 6 iron hosel. The etching is not backfilled so I had to look carefully to see the serial code.

They aren't mismatched. The 4 thru 7 are designed that way to provide the forgiveness some of us so desperately need. Fear not, it works! Enjoy your new sticks.

Congrats on the new iron set. I've had them for two seasons and absolutely love them. The are not mismatched. The "M" or max impact technology and is only on the long irons through 7.

Unwrap those babies and enjoy!

That’s the way the set comes. The long irons have a different design than the short irons. Look again for a serial number on the hosel of the longest iron in the set.

No serial number. Did you purchase these from a Titleist account? Those definitely say T300 on the clubs. My concern would be where you purchased those. Most clubs have a serial number for a reason. They look real from the back but there are so many great fakes out there. You can always call Titleist customer service and ask them about your concerns. Maybe they can help you out. Best advice. Call this number and ask for customer service regarding clubs: : 1-800-225-8500

Off the rack sets from the big box store only have it on the 6 iron.
The 8-W don't have the M logo because they don't have the Max Impact technology. They are designed that way.

I also had the same feeling with all the great fakes around.
But yes, the club backs are different.
The longest iron in the set (my 4 iron) has a serial number, but when I enter it onto Titleist website it says it "We were unable to locate any club details associated with serial number 3093075. Please contact 1-888-TITLEIST for further assistance"
That doesn't really help me because I'm not in the USA.

I say you have no worries, play them with pride. I have the same set, not upgrading until next year.