For the curious, the Metabones "Ultra" 0.71x Speedbooster optical cell can be fitted within the throat of the "big" URSA camera.

I initially sought only to add one into the optical path for my Nikon F-Mount fitment. The optical cell requires the flange face of the mount to be moved around 4.2mm rearwards for correct focus to be maintained to the sensor.

The Nikon mount I made is narrow enough to fit rearwards inside of the camera body. I initially thought that the BM PL-Mount could not be used with the Speedbooster in place however I recently examined the possibility and have found that with some material machined off the rear of the PL-Mount locating tail and from the rear of the clamp ring and "ears" for tightening it, this is doable - JUST.

The position of the Speedbooster optical cell in the throat of the camera reduces the number of PL-Mount lenses which can be used. Some lenses will penetrate inside the mount too far rearwards and will clash with the Speedbooster optical cell and damage it and the rear of the lens. Some vintage lenses also will have an image circle too small to be used with the Speedbooster in place.

Just a little hint for folk to do their own fixit if you happen to have an URSA with a focal reducer fitted in the throat. My arrangement has a Caldwell (speedbooster) in the throat of a "big" URSA.

A few Lucadapters may have been ordered with the optional Caldwell optical cell for URSA and URSA Mini.

If you dismantle for a clean, be aware that the Caldwell optical cell can be refitted back to front and it will still work.

Rather it will seem to work but you will soon find the collimation (what some call backfocus) of the lenses is way out.

The long lenses will work. The zooms will go nuts, The ultrawides in the ballpark 12mm to 25mm will be way off infinity focus and be very furry in the corners as well.

I have just sent myself a little insane thinking I had messed my shimming up. I pulled things to bits and totally missed the Calwell optical cell having been reversed.

While I was in the guts of the turret stomping about, I cleaned out and replaced the thermal compound where needed.

I had swapped out a sensor from an EF-Mount turret into a PL-Mount turret and found the shimming brought across from the EF-Mount turret was off for the PL-Mount turret, so used the original shim set for the PL-Mount turret and all was well or so it seemed.

It is probably good practice to take notes, even phone photos as well as systematically laying out parts.