Inserts are blank when they are installed in a playfield. The black legend is part of the silk screen process so no one "screwed up" It was intentional for whatever reason. Unless the playfield was restored with some kind of sticker set (legends worn off all/most inserts).

Zone 4 and 6 numbers are the wrong font. That sun is also looking pretty demonic. Probably done afterward?

I'm guessing it was an op.

Who knows why ops do anything!

Anybody can make insert decals, it's not that difficult. My guess is they got worn, the op whipped up some home made decals.....BAM! Project done

Hehe. + and +. That's lazy.

This looks like all the original inserts were removed from the game and either had the original printing sanded off, or were replaced with blanks, then had new decals/stickers put on. Half the thickness of the black outline around each insert should be on the playfield itself, and half on the insert; that half is missing here (in addition to the strange numerals and the goofy smiley face on the sun). I'd love to know what the story here was.

Edit: I bet these were Chartpak dry-transfer letters.