RE: "Thunderbird" valve covers?
As Alexander stated Ford used both styles for the 58 bird. I have not figured out yet when the change over occured but based on info supplied here and some of the ones I own would place it somewhere late in production. I have 58's with both types.
The plain valve cover plug wire holder are too big to accept the wire without some sort of grommet/built in geommet as part of the wire. The openings are too big.
58's are one of those unique years for the bird where Ford tried many new ideas, and several of them flopped.
Sun Prairie, Wi.