If it looks like a Ford 9" with a third member that bolts in from the front, it's probably an 8 inch. Very common on Mavericks, looks like this one from a 75 Maverick 250 C4:
Most Maverick/Comet 8" came with 2 common gear ratios: 2.79 for cars with no A/C and no P/S, and 3.00:1 for cars with the options.
9" were pretty rare in Mavericks, I think exclusively with V8 cars, but I'm not sure. I think a Maverick Grabber or Comet GT still had 8" rear if it had the 250 and not the 302.
If it is a 9", it probably has been swapped out in the past from an 8".
The way to tell the difference between a 9" and an 8" is to look for the flat spot on the top of the housing. If the diff and the 3rd member are completely round, it's a 9"
if it has a flat section then it's an 8". Here they are side by side, 8 on the right:
The 2 other very common Ford rear ends have sheet metal covers on the back and look a lot like a Dana or GM rear end. They are the 7.5 used in pre-66 Falcons and Comets. And in the 70's they came back in 4 cyl. Mustang II's, Pintos, and Bobcats. very light duty. These may be found in other, older model, very light duty, applications.
The 8.8's are on later 80's model and up (modern) Mustangs, Thunderbirds, and Rangers, Explorers, Mark VII
Here's a pic of the 8.8 on the left next to the older, smaller 7.5 on the right.: