Iterate through shape's master shapes

Hi,
I prepared master slide M1 with one special shape. I put into that shape
some information ( in tags property). Now I am creating new slides A,B,C and
I use that one master slide M1 for this and I add new shapes on that new
slides A, B, C.
What I want to achieve is go through all new slides, then get shape which
belongs to master slide and change its text to something different on each
slide. But in Shapes collection in current slide there is no master's slide.
So I check
myPresentation.Slides[i].Master.Shapes[j] and I found my special shape with
tag data inside. Then I change text ... and it is changed on each slides not
only current one. Can I set master slide shape's text to different values ?

Regards,
Piotr