I have several sets of old Wilson staff blades from the early and late 60’s. Have the kind of grips you are describing, I think. they would be non conforming today, as I understand the rule. We have nice leather wrap grips (Grip Master) and it would probably be the best current option to replace, and they are conforming. None come with a rib, but you could order the under listing and the wrap, and create a rib the old fashion way. A piece of coat hanger on the back of the under listing, then wrap the leather around it and there you go. Probably not conforming either, but it does create a ribbed leather wrap. We have our reminder ribs (GW0017) that could placed on the under listing instead of the coat hanger. Whether a grip like this is conforming or not depends on how pronounced the rib is. USGA kind of gray is this area. I have seen some grips that appear to be none conforming actually be considered conforming by the USGA and also had grip rib set ups that appeared to clearly be conforming that they ruled non-conforming.
So, there are some options with regards to leather and ribs, but no flat area under the heel.
Britt Lindsey