Don't panic. The little steel barrel is an elastic spacer to keep the two prisms in the right position. The prisms work in only one position, so you can easily "try & check" without problems:
- the larger prism must be inserted first with the "roof" side down
- the barrel is positioned in the spaces between the two prisms in the upper side
- the second triangular prism must be inserted later, in a way that one surface has contact with the first prism, second surface is in contact with the barrel, and the outer surface is perpendicular to the optical axis of the viewer. There is only one position to have this condition.
Remember that even in perfect assembly, you haven't a perfectly circular view, there is always a little cut in the upper side, but anyway is outside the square field.
Hope to help you!
