I've seen photos of the undersides of some F-100's and comparing rear springs mine has several more than stock. The PO said he added extras but what is stock I count 9 leaf's and no more room on the U-bolts for more.
Did he try to make an F-250 out of it? It has some really big air shocks back there also.
Here is a photo, check it out.

 

It depended on the load capacity the truck was set up for from the factory. There were 3 choices on 1965 F100 2x4.

Standard - 950 lbs - 5 leaves
Option 1 - 1250 lbs - 6 leaves
Option 2 - 1650 lbs - 9 leaves

There was also an auxiliary rear spring available with a 380 lbs capacity. This option was not available with the 1650 lbs rear spring option.

If you got the auxiliary springs then the U-bolts point downwards, otherwise, upwards.

 

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