The cable may have had a problem at one time, and someone just removed the whole mess. Perhaps they were having trouble getting the key out, due to the cable not working properly. Who knows?
Are you saying that your steering column does NOT have a lever on the bottom of it? Could you look closely, and see if possibly it has been cut off?
It's also possible that someone has replaced the mastjacket(column), and the replacement didn't have the lever. It's hard to say what happened.
It is a good safety feature to have, as it keeps the car from being shifted out of Park(auto) or Reverse(standard) unless the key is on, which mean you can't get the key OUT unless the trans is in Park(or reverse). Should you try to get all of the parts to "fix" it? Maybe...it's a good idea when it works properly. The parts may all be available fairly reasonable, price-wise....might need to take a look on fleece-bay, or check with some of our Sponsoring Vendors, to see what is available.

Joel,

I wonder....here's a thought. Is it possible your car was an "Export" car? Perhaps to Canada maybe? I'm not saying I know, 'cause I don't, but....Export cars were not always required to have the same "Safety" equipment US cars had. That could explain why there is no evidence of any of it being there.
Just a thought...

Not a bad thought JA.....I remember questioning the PO and even though he was not the OO the title trail from the dealer he bought it from says

I think the system is otay on automatics, since it requires you to put it in Park to remove the key. It works the same way on standards, but most folks already have the car in a gear when they shut it down.
It is also a good anti-theft mechanism...if you can't move the shifter, it's harder to steal the car.
It's basically 6 one way, 1/2 dozen the other. You might try contacting a place like Paragon, or even Larry @ Corvette Specialties, and see if those places might have the cable/brackets for the trans, used.

I also have a 1978 Automatic CANADIAN coupe and it does not have the transmission interlock cable. Took some getting used to but there is no need to add it if it is missing.

vettelang, does your car have the bracket at the lower end of the steering column for the cable?(see second pic above)
I'm curious to see if they used the bracket whether or not it had the rest of the parts for the interlock.

The bracket is on the firewall, but there is no lever coming out of the steering column. Saved money on a cheaper column and no cables.

I've followed your posts on the interlock cable. I have trouble getting out of reverse first thing in the morning ('73 4 speed car). And it doesn't always want to go into first.I've lubed every moving joint I can touch, but I'm thinking the cable is frozen. My column doesn't always lock when I put it in reverse, either.

I have a 78 SA and it doesn't have the interlock cable either.  It's first home was Nuremberg Germany (according to the build sheet). If it had a interlock cable it would have saved me some heart ache. A couple of years ago I was doing something on the center console and put it in neutral, forgot about it.  Later I opened the garage door with my remote and heard a weird sound.  The car had rolled back and the garage door handle had caught the rear bumper and put a big gouge in it. Oh yea don't see any bracket or where a bracket may have been.