After assembling yet another Pak-40, I should make a note about part A18 since the instructions are not clear.
The gun sight (part A18) should line up with the square view port on the gun shield (part A20). I recommend that you glue the gun shield on last after you assemble all the other parts in step 1, but if you want to add part A18 after gluing part A20, have a pair of tweezers handy.
The square shaped view port on the gun shield (A20) is made in a closed position (shutter is closed), but with care it can be modified to be in an open position. A true die-hard modeller would cut out the square view port on part A20 if the gun is being built as a deployed AT-Gun ready for action.