The "J Rendenbach" stamp is what's stamped on the soles of all AE shell cordovan shoes (unless it's an MTO where you specified something other than Rendenbach soles). The Horween stamp you are thinking of is probably the one found on the back of the shell cordovan leather itself. You would not see this in a shoe unless it's unlined on the inside.
Newer JR soles do have a Horween stamp on them as well (see Tifosi's pictures of his MTO's above). That's just something they added more recently. It has nothing to do with the authenticity of your cordovan shoes. Besides, it is right at the ball of the foot, where it will wear off in no time anyway.
I stand corrected! I never even realized that my last pair of AE shells had the Horween stamp. As MWS noted, it disappears after a few wears anyway.
I had even taken a picture of the sole before, haha. Guess I shouldn't comment on stuff I don't really know about.