This information is for qualified technicians only; do not attempt the replacement if you do not have experience in handling the glass properly

For the Cooke S4 (/i) 100mm the glass part number is 99-1492, it has a flat side facing the film plane and a convex curve toward the front of the lens.

One word of caution, the third glass from the rear is a thick element held by a very fragile thin glass flange. Refrain from disturbing any more glass and limit your repair to the rear element only.