I was given excellent advice years ago:
"Never buy a perf 12 W/F coil without a certificate".
There are far too many fakes. That's why I only collect them as already certified joint (or line) pairs. I've had enough of my own pairs come back from PSE as fakes. In addition to deliberate fakes, those

single line watermarks can almost completely miss a single stamp on the sheet. Much easier to see with multiples.