Rear element of Cooke Series XV triple convertible

Good day,

I recently picked up a single coated Cooke series XV triple convertible mounted in a Betax No. 5 shutter. The rear element seems permanently attached to something that has a larger diameter than the front element and so the front element cannot be put in place of the rear element when used alone. Is this just how the somewhat later Cookes were constructed? Is the front element to be used in front of the shutter? Normally, when used alone, the single elements are to be placed behind the shutter (looking toward the subject). That being said, though, I have previously used the front element of my Wolly triple convertible in front of the shutter (rear element removed) and the negatives turned out just fine. Maybe Jim Galli can chime in on this one...

Thanks very much,

Markus