It is possible but I have never heard of anyone getting a boot from yaton. I have bought most of my collection from him and never encountered one. If the numbers on the mask roms match the number for the game in the master list i'd say it's some sort of repair. Check that first or post some pics of the boards so we can see them.
Yaton checks his carts pretty thoroughly, worst I've gotten from him is bad fake labels. Lots of bad fake labels. The games I got from him a couple years ago came with the regular battered MVS labels (serials cut off or permanent markered, stains, etc.) now almost all his games seem to have cartoonish homebrew labels or slightly blurry inkjet printed on photo paper copies of good original labels. Overall I'd give him top ratings for selection and selling original carts. He does check inside, that's why he puts in his auctions the Original game - Fake label bit.
I have 1 boot that I can't remember if I bought from yaton or not, but I am 99% sure I got it elsewhere on ebay. I didn't open the cart until long after I bought it (metal slug 1), but it works perfect.
I do know that Yaton is a member here and occasionally reads the tech forum, but he does not sell here. I pm'ed him once after he made a post and I put 2+2 together, right after I had bought several carts on ebay, and he pm'ed me back to say that my carts had shipped and that he wasn't active on the forums much but occasionally checked for answers to tech problems.
cheers,
-DT