The Mariners catchers this year have a .466 OPS. They will likely end up being the worst bring group of catchers since the 60's which is as far as the baseball reference play index goes according to an article on Fangraphs. The 1968 Giants SS have the honor of the worst OPS for a position .438.
Just looked it up.
Hal Lanier 518 PA, .206/.222/.239 wRC+ 39
Don Mason 20 PA .158/.200/.158 wRC+ 11
Nate Oliver 78 PA .178/.189/.250 wRC+ 18