My Evel Knievel (Bally 1977) has a full mylar overlay covering the playfield, except for the ball shooter lane.
Go to the top 100 machines on this site, then go to 'what comes after 100" (down the bottom of the page.
Go to "126" Evel Knievel, then to Gallery (machine pics).
The first 5 pictures are of my machine, which shows the playfield.
The game play does not alter and the playfield is a breeze to clean.
I can only think that the last owner applied it, to protect the playfield, considering its age, it is in A1 condition.
It does not get played a lot, so i presume that if i ever intended selling it, i could remove the mylar and have a pretty much, pristine playfield (mylar removal however can be a problem)
I have some better pics of the playfield if you want to see them, let me know and i will post/email them to you.