I have this kit in my huge stash of 5 kits. Looking at the instructions, the light blue B3 (two B sprues) looks like the left side of the breach block and the whole block can be positioned open or closed. B38 looks like it sits on the bottom of the gun rails and B32 is the rear part of this rail, joining together B28 and B29.
This instructions are a freaking nightmare to read, too much going on in each step.
Here is a link to Andy deafpanzer's Marder II and he has a photo of this radio 8 photos down. Bill Plunk sent him a diagraph.
cs.finescale.com/.../136898.aspx
He also has a photo of the gun rails and you can see part B32 towards the rear of the rails.
Here is a website with a photo that sells this radio: www.tigermodels.com/.../Details.php
Good luck with this kit, it scares me too much to start.