This oval plug showed up I believe for the 1983 model year, or whenever the feedback carbs showed up. A large wiring harness passed through the firewall (also for the later 1985-1986 5.0 EFI equipped trucks). If you happen to get a truck that didn't have a feedback carb, then there was a large oval rubber plug put in this hole where the EFI/Feedback harness would normally be.
My 1984 F150 had one from the factory:
These oval plugs are really hard to find, and they're desirable for the guys that swap out a feedback carb for a regular carb with DSII ignition. If you remove this harness, you need that oval plug to fill the hole.
I used to grab them at the junkyard when I came across them, but I ended up mailing them out to guys that needed them.
Not sure what the part number is...it's surely to be a junkyard only item.
Do you have dimensions?
This doesn't exist on my Bricknose...