Thanks to John Mummert's video, I now know the difference between Y-block timing covers.

I was offered a free ECZ timing cover for my 55 Tbird. I knew that not all covers are the same, but I really didn't know what to look for. The ECZ timing cover is for full-size Fords equipped with Y-blocks; it is not correct for 55-57 Tbirds or trucks. However, Tbirds and trucks can use the same timing cover. 

My 55 Tbird has a 56 ECZ-A block. My timing cover is a C2TE used on 62-64 trucks.