If the drill has reverse, it will definitely have a screw inside the chuck and it will definitely be left handed.
It probably doesn't reverse though, from the age of it.
But you still have to open the chuck to check!!
If it has one, it has to come out first!!
After the week long (2 is better) soak in penetrant, put a 3/8" hex key in the jaws (the short side) and cinch the jaws down tight.
Now take a light hammer and knock the chuck loose by striking the long leg of the allen key in a counterclockwise manner.
You want lightweight hammer (8-10oz) and a very fast, hard blow.
or 50 blows.
But it will eventually relent, and its the only way without damaging a whole lot of stuff.
Bigger hammer is no good. Impact wrench, definitely no good!
fast hard impact from a light hammer will work, eventually
yours Scott