Sets may only have a serial number on the lowest numeric iron.

All sets sold new through authorized dealers in North America are assembled in Carlsbad, CA. With components imported from other countries.

I don't know if Titlest assembles clubs for East Asia in Taiwan. In any event, if purchased in the US or Canada, someone brought them from overseas or they are counterfeit. If they are new and the price is under $200 a club, ...probably too good to be true.

If you aren't in North America, that would be good to know.i

Serial number normally on the six iron. We on here are loyalists like yourself might contact Titleist customer service for better evaluation.

Did some resarch on previous models, below, looks like the 2021 T100 model. Hope you got a real good deal. If they are real. I am very leary about purchasing online, especially thru eBay. Too many fakes on line. Just saying, with respect.

For what it's worth, I have those model irons and I have matching serial numbers on every single club 4-GW. My clubs were previously built custom fit from a fitter, not an off the rack set.
I know that off the shelf clubs sometimes will have only a serial number on the 6 iron and not all of them.
They possibly could have bought a 5-PW set and purchased the 4 iron separately. If they were purchased grey market, it could be different than clubs built in Carlsbad.