After searching for about 1/2 hour, we finally found the C. ferruginea right below a limestone hill at Bau in Sarawak. We passed by that area for a couple of times during our last trip but did not imagined that any cryptocoryne would be found there. Actually if I had done my homework more thoroughly, I would have found photographs of that area in websites of other cryptocoryne hunters which had visited there before.
You probably could see the limestone at the background in the photograph below. The cryptocorynes were growing along the edges of a pool of water under the limestone hill.
Both submersed and emersed specimens could be found there.
The leaves of the Cryptocoryne ferruginea there were quite wide and were green in colour both on the upper and under sides. The leaves also had dark green markings.