Surplus Shed 38mm Erfle eyepiece focal length error?

PS I also measured my 6mm and 9mm Synta UltraWide EPs with and without
their built-in Barlow (which can be easily unscrewed from the eyepiece
body). The 6mm became a 15mm and the 9mm became a 16mm.

Have you ever tried swapping the "built-in Barlows" between the 6 and the 9? I
wondered if the only difference was the "built in barlow" but after swapping
them, it is clear the entire eyepiece is different.

Also, I believe these are not barlows but maybe Smyth (?) lenses as the view
without them is quite distorted.

jon