I'm in the process of putting my 260's interior back together with seats I purchased from someone that said that they came out of a 260. The rear bolts line up but the seats have a tab that tilts down at 60 degrees in the front that doesn't line up with anything. I'm likely going to just bend a steel bar and drill a couple of holes in it to make it fit.

Is there a part missing or is this just a variation in seat design?

Those are later 280 rails. 77-78 only  I believe.

Yup, those rails are 77-78. The 74.5 to 76 have 60 degree tabs on both ends, like the front tab on those. 240 rails have studs on both ends that fit your holes. I have lots of the 240 rails, let me know if you want a set. Send me a PM. 

I have the same on my 11/75 made 1976 280,  they probably came from a later 260 and you have an early 260.

My 260 is a 4/74 manufactured so the source vehicle must of been a late build. 

Does anyone see a problem in just bolting a 60 degree metal tab or should I replace the rails? I unfortunately have lost access to my metal brake until who know's when. I'll likely have to purchase 45 degree angle braces and hammer them to get 60 degrees.

On a side note, I found one of the nylon bolt-shaped spacers used to adjust the seat height. It was hidden beneath the 1980's vintage carpet. Earlier, I found a pack of camel's in one of the tool compartments that has been hidden by the 1980's carpet. This car is a time capsule. 

Stay Safe Everyone and Thank You for the Help. 

Just replace the rails - get the correct rails for your original mounts.

I purchased the correct rails for my 260. Thanks for all the input.