Normally there is no glass coating to be found on/over the seam, just the adhesive compound.
I have owned 5 Beneteau's type "First" ranging from 21 feet and up until 42 feet and never found anything as described. My boats have been ashore every winter period and we always inspected the underwater part(s) painstakingly. When necessary all defaults were treated during the winter.
Deteriorated washers can be a caused by (dirty) bilge water, even stainless steel can be affected by oxygenless water.
A cast iron keel and also the alloyed lead ones normally carry a special primer coating on the outside, before being painted (I believe it is a resin based primer and it feels like teflon, you can ask Beneteau, Z.I. les Mares, B.P. 66, 85270 St-Hilaire-de-Riez, FRANCE, tel. 51.55.53.82, telex 711 497 F).
It is my guess that a previous owner has possibly tried to gain kind of streamlining from hull bottom to keel surface, maybe for not trusting the seam and therefore covering it.
As you already removed some of the glass coating I suggest to go on all around the seam in order to detect any rust traces that eventually can indicate whether the keel bolts are healthy or not. If such rust traces are clearly visible it is advisable to remove the keel and after inspecting and/or renewing the bolts, seal the seam again, put on primer, paint and anti fouling.
The picture did not show any rust traces, only some traces of coarse grinding, so pay attention on the keel surface and the inside surface of the glass coating for any rust signs.
Don't be afraid, removing the keel is not very difficult and eventually renewing only takes a certain time together with adequate tools.
