The built in IR "window" is screwed into the front of the camera body, the black bit at the front of the camera in this pic:
http://s4db2ab52c6d7a.img.gostorego..../qhy5-ii_2.jpg
If the filter were removed in this picture there would be no black bit at the front and you would just have the coloured body.
The thread on the front of the filter is much too small to take the extension tube so the filter must be removed in order to attach the extension tube to the camera.