It's a locator to key the connector. Not sure why the additional key was necessary. The locking wings and blocked off terminals of what would normally be a six terminal connector are pretty darn effective.
The 81-86 & 87-91 trucks use the same connector 12004101. There is another similar connector 12004139 with a differently placed locator notch.
Look at the connector and you'll see a notch toward the center for the locator. The 12004139 connector was used on some of the 89, 90, & 91 RV trucks. Apparently Delco is only making one TBI style switch now and we have to deal with it.
The 12004139 connector looks like this. Notice the additional notch toward the centre on the corner socket. The more common 12004101 has the notch in the centre terminal on the same side.
It's not the end of the world. That switch will still work just fine you just have some extra work to fit the switch.
If your switch is setup for the 12004139 connector You'll need to extract the terminal from the connector shell and make a notch in the plastic to mate with the odd locator bump then re-install the terminal. You can do it with a small saw file. I have done it to make an oddball switch work and it works just fine. I didn't know at the time why the switch I got was slightly different and I bought some after that had the old locator bump. One benefit is you can use a switch with either locator when it's done.