The lintel will be made of steel.
Back then, that was the only material strong enough (short of a
Stonehenge size slab straddling across the two walls) to support the
weight of the masonry above the window.
Check with a small magnet.
There were also "cast lintels" in use by the 1960's. This was a form in
which steel rebars would set and the concrete poured around them.
Nowadays, that's called "reinforced concrete". The idea behind
reinforced concrete is as simple as the concept of drywall. In order
for drywall to bend, the paper on one slide or the other has to stretch
to accomodate the new shape. But, since paper is quite strong in
tension (given't it's weight), dryall is actually very rigid
(considering what It's made of).
Exactly the same principle applies to reinforced concrete.
By embedding layers of rebar in a concrete slab just a few inches from
the top and the bottom of that slab, then in order for that concrete
slab to bend, then the rebar on either side of the concrete slab has to
stretch, and since steel is very strong in tension, the steel rebar in a
properly designed slab won't allow the slab to bend far enough to cause
the concrete to crack from excessive stretching on the side of the slab
that's experiencing tension.
So, I expect your lintel is steel, and you can check it with a magnet,
but it may be made of reinforced concrete. I just don't know when
reinforced concrete lentils first started being used. I know my
building was built in 1960, and it has steel reinforced concrete lintels
over the apartment door ways, but only steel lintels over the apartment
windows. That's only because the door way lintels are visible as long
concrete blocks over the door ways, and that matches the the concrete
blocks making up the hallway walls.
Dunno if this helps.