Actually Garns are the only boilers I've heard having anode rods in them - but I haven't seen a lot of OWBs.
Garn actually stopped using them a couple years ago I think - they were having problems with them disintegrating then the crud getting into the system & messing circs etc. up. They are big on regular annual water testing & treatment though - likely the best course of action for looking after boiler internals.