Hi Tom,
Yes these discs were only fitted to the early Liberators (LB-30, LB-30A, LB-30B, GR I, B II, C-87). When the later models came out they had the ovular cowling that had the cooler intakes at the sides.
Elsewhere have suggest a form or deflector/director which speeds up airflow to aid cooling. One thing against this is that the CAF LB-30 Diamond Lil, originally had these discs, but now doesn't. Unless the CAF have changed to different spec engines (which don't need these discs), then it should still need the discs, so why aren't they still fitted?
Shessi