Ok so I hope this will help you as I remember the first time I tried to take mine off and I knew where the key was. it is in the lower right side cover.
So my problem was I could not turn the key. But the answer is you push down on the seat and then turn the key. it takes pressure off the locking mechanism.
Now there are a couple different seat styles the dual touring style that started on the Classic models.
with or without backrest does not matter.
Here is a shot of the bottom and latching devices.
Here is the dual/single which mostly came on the Standards like yours
It has only one latch controlled by the key the pillon is bolted to the rear frame. You can go single or two up with this seat.
Now pushing the seat down when turning the key not only takes pressure off the latch (below closed and locked)
Below open with key turned
The key is pushing down takes the pressure off and stops the oh ***wit I just broke my key!!!!!!!!!!!
Hope this helps any other questions ask away.