Rob, if you change the cell spacing you'll change the focal length and undo all the work Pacific Optical's designers did to make the lens produce good image quality.

My USAF data sheets list three different 12"/3.8 Pacific Optical lenses, two designed to cover 6x6 and one for 9" x 9". None has dimensions that match yours perfectly. My data sheets are incomplete so there may well have been more 12"/3.8 designs from Pacific Optical.

Lenses from aerial cameras are often poisoned gifts. Most are hard to put to use, some aren't very useful for general photography.