This "non-existing" pin is just a key that allowed to position properly the cable. Nowadays, like in your upper photo, you have part of the plastic removed, where the key from the cable goes. It allows you to know which side of the cable should be upper-facing, because otherwise, your keyed-cable wouldn't fit to the socket. Earlier, the cables didn't have such key in the plug, instead of it, they had a missing pin in the socket, or filled one in the cable. Sometimes they didn't have any key, so the cable could be plugged both ways, so you had to properly find the first pin, marked in the cable by red color, and match it to the both sockets. There's nice description of differences between the cables: informit.com/articles/article.aspx?p=2028834&seqNum=3 Upgrading and Repairing PCs: The ATA/IDE Interface
The keyed cable is "80-conductor cable" where the unkeyed one is "40-conductor cable". The 80 one is newer standard of ATA, where the 40 is older, unkeyed one. You should search for "80 conductor ATA cable".
PS: Also worth mentioning, the increase in the conductors in the cable, without changing the pins in the cable is due to the additional conductors being only grounds, used to decrease the interference between signals by inserting the grounded wires between every signal.