HI,
As Michael says you might have the wrong bolt in hand.
You may need a longer regular hex head bolt if that one in your hand is too short. You will also need to use a flat washer under the hex head to fill in across the slot for a better surface contact.
The rear bracket can be knocked down a bit lower it as it swivels like the front adjuster one.

Since this one is shorter than the rear bolt that goes there, it's not the right bolt.
Does it have a smooth, oval round head as the front bolt does?
If it does, you have the one for the front of the other compressor. It's missing from that adjuster bracket.

It will have a low profile head with a square neck on the body underneath.
This type bolt is called a "carriage bolt" in our hardware stores and usually has a coarse thread count per inch. It was used to pull down into wood of a carriage bed or truck bed to let cargo slide over. The square also fits square holes in metal or in this a case a slot of a bracket. Some of the alternator's housings have square holes.

The front bolt needs to be a carriage bolt, so it clears in the space underneath the adjustment screw.
The square neck fits into the slot so you only have to use one wrench to loosen or tighten the bolts nut up is another advantage.

FYI
Fords were made of wood in many places. He recycled his vendors oak pallets and machinery crates.
Back in the day of Fords assembly lines, one man laid under the carriages bolting on the wood boards. Bet he had a "speed wrench!
Like ones we have now that have a bend in the middle tool of which I seldom used anymore due to air tools. You know the one that looks like a carpenters auger's bit brace! It's good for the bolts for the plastic belly pan.

Just thinking again!
Probably where the idea of "rolling creepers," that we use in our nice driveways, "really" did come from the 1910's.
It was later, just before World War Two, that an employee or relative of either place worked for the Dodge Brothers too! The Dodge brothers were original parts suppliers to many of the car manufacturers.
Yep ... That's were we got that term "Jeepers Creepers."
Yep ... I remember the "Little Rascals" and "Our Gang" episodes using those words!

Hmm, after all these innovations, we still think we have horses powering our (car)riages around!
Remind me again, How far away are we from that apple tree that started all this, anyhow? (:-)

Hope you can find all your pieces! I end up finding the right box ... eventually? (:-)

Phil