G'day Dave,
Yup, your right there is a guide each side of the deck pulley as well as the bolt at the front. My apologies, I was working from memory- too tired and lazy to walk out to the shed last night!
The absence of the forward keeper could be part of your problem but my gut feeling is that it's not a huge deal. It should be there and popping it in will cover that base. Once belts start rolling over for whatever reason ( just an old belt or some other underlying problem), it'll keep doing it and need replacing. It's possible there is another problem such as looseness in the cutter deck mounting or height adjustment mechanism or weak spring tension but a new belt is the place to start by the sounds of it.
Hope this helps.
Cheers,
