dual batteries

My local GM dealer offers two left side (auxiliary) battery trays for my 85 longbed with a 305H motor; one is just described as "LH" (left hand) and the other is listed as "diesel engine". Does anyone know what the difference is? I'm thinking maybe the diesel one is set up for a bigger battery? I like to run two batteries with an isolator; it's nice to be able to jumpstart yourself from the cab or have two batteries to jump someone else in need...