There was only the voltage limiter at cluster plate.
If there's a voltage limiter on the cluster, and the car is "untouched", then it was never equipped with the
optional "low fuel" alert.
The low fuel relay has a voltage limiter in it, so there's no reason for the voltage limiter on the cluster. Note that in the picture from the FSM that you have posted, you've highlighted in yellow "with low fuel signal, remove voltage limiter".
So, you aren't missing it, it was never there to start with.
A picture of the back of the cluster from my '70 300. Note the empty slots for the voltage limiter.
And a picture of the relay, showing location etc. from my car.
Pics (from the internet) showing the internals. You can see the low fuel contacts and the voltage limiter contacts (called pulsed on the pic"
The cluster will have the "low fuel" light location regardless of how it was optioned.
If you really want to add it, you'll need the relay along with the harness. You'll have to remove the existing voltage limiter.