Startrekk Saddle Question

I am not 100% familiar with this range. So I welcome any input readers may have to the following query.

Startrekk Dressage.

My changeable pommel is D3 –
I have completely no idea what width it is (in cm) – could you please
explain it to me please? Gullets for this saddle are defined as
Gullet: K1, K2, K3, K4, K5. There is no “D” as in my saddle… I’ve
got problems also with ascertainment, if this one (D3) is suitable for
my horse. I don’t want it to be too wide or too narrow.

My last question is about the small plastic elements hidden between panels
and seat. I’ve never seen anything like this in regular saddles. Could
you please tell me how to properly use them? Can I remove them and if so
– what then will happened?