The block is not a massive one, it's hollow. (U pipe), But too narrow as a cartridge guide. As you can see there is a drill hole that fits exactly or replaces the original magazine blocking system (you know the hook with push button in front of the trigger). The whole system connects the bottom of the receiver with the top of the trigger guide. There are traces of inside wood removal to fit this whole block. It lays to deep for a single action rifle so there there would be doubts about that theory. 

Again wood was removed for fitting this block. This picture shows an "S" cartridge (found in Verdun) inside the block. It took force to get it to fit. On the front of the block is a beautiful "F" mark.