What is / are the visual differences between the "B" and the "C"?

I have the Tankograd/Marsh Gelbart book, along with the Osprey, Mouse House, Darlington, Concord books. These books do not mention the differences between the sub models.
The closest reference I have come across is the web site "Israeli Weapons".
http://www.israeli-weapons.com/weapons/vehicles/tanks/merkava/MerkavaMk2.html

Hey Tony,
Tough call, even this site isn't too sure:
http://idfmodelling.free.fr/merkava2.html
If I had to guess, maybe the presence of upper turret armour and additional armour over the engine deck? (both have Mk.III type skirts, bins on the rear hull, smoke grenade launchers front side turret, periscope in new Commander's hatch)
Maybe ask the guys at the IDF Forum:
http://www.idf-armour-group.org/
Lots of knowledgable types there, when they answer

Cheers,
Jim