williams fp sights on a rws 34 (airgun)

[sarto]

Hello to everyone.

I own a rws 34 like this
http://www.pyramydair.com/p/rws-34-air-rifle.shtml

I would like to upgrade the standard rear sight with a peep sight.
Looking on the Williams webpage, I saw that they offer 2 model for the airguns:
- FP AG for the guns with high line of sight
- FP GR for the guns with low line of sight

As you can see in the photo in the link I posted, the front sight isn't easily replaceable.

So the question is:
Does anyone here know what model is the right one?
There is a simply method to decide?

Thanks for the answers.

SARTo