Icosahedal Kaleidoscope by George Beck

Hi,

I came across a guide on how to construct a dodecahedron reflection from 3 mirror pane cut to shape, but I cannot make sense of it.

My guess is that mirror shape 1 is C1-C-Iy-By-C1
mirror shape 2 is C1-C-M-A1-C1
mirror shape 3 is A-M-Ix-Bx-A

Can anybody help?