There's not really a "factory" weight for most BC's. Soft weights will work, as will hard weights that will fit inside the pockets and still allow them to be removed easily. I doubt it came with any weight from the factory originally.
If it's a back-inflate model and has the rear weight pockets, start with a 60/40 weight distribution f/r.
Is this an old model? a quick google search returned no pictures. If it is an older model, you may want to get it serviced at a Mares dealer before using it.
This may not apply to your BC, but some Mares models with the removable weight pockets had issues with them falling out in rough conditions. A friend who had an older Mares lost a pocket on two different dives over about a 3 year span. Hers had sort of a red plastic retaining clip. And I found one on the bottom off Roatan last year - looked in like new condition.