Hi Microfish,
For your question, the reason is MD04/MS04 used different calculating method in opening stock, for MS04 the planning scenario is taken into account during planning run. You should double check the planning scenario which was used for long-term planning, what kind of value was chosen at field of "Opening stock"? If tha value were "2" - Plant stock at time of planning, you would see the same stock quantity in MD04/MS04 before MB1B+315.
Best regards
Tao