If there's no shaped compartment for the forearm try fitting it onto the barrels and see if you can put it in the barrels partician, many shotguns and Double Rifles were stored in their case that way. The large rectangular shape where the picture shows the forearm may have been used for boxes of ammunition and the small square space will have been for an oil container usually made of metal and fitted with a screw off cap which may or may not have a needle fitted to the underside of the cap to place oil accurately in the lock or elsewhere. There are so many choices, you may have to find someone with a similar case by the same maker that has all the accessories to be sure what went where.
Harry
