David,
the Ultropak lens is oftenly missing - and more oftenly overlooked. If you look from behind into the UO and you discover a tiny glass plate, you have found that lens. It has not a big focal length because it just has to adjust 185mm to 223mm tube length. Therefore it appears like a protective glass plate.
By the way, it is not a big issue, if it's missing, especially with low power objectives.
Cordilament
Wolfgang