Harvey,
Good eye on the guard, it is not original. That is not the anomaly I meant but still impressive you knew it was not the right guard.
Chestnut can be used as can most all nut woods. Some are more neutral than others and some have tighter pores making them a better choice like cherry, black walnut and maple. Lumber from live oaks and water oaks I have found make very nice boards and as long as you keep them maintained they work perfectly and chestnut would be in that same pore type.
The liquid on those boards is mineral oil, it is applied until the board won't soak any more up. It fills the pores and protects the wood. It is the only oil to use on one. I use something called spoonbutter to maintain mine, it is a mix of mineral oil and beeswax. Rub in, let sit, wipe off and done. If you do that monthly the boards will outlast our grandchildren.