hi bruce,
the mount is glass filled nylon or similar. the 4046 i assume refers to the engine size range it is made for and the "L" might refer to it's arm length from base to tip, because it seems pretty long to me... the backplate of the engine is 1-3/4 inches away from the back faceof the mount,... where it would sitagainst the firewall....
there's a flat shelf between the arms extendingforward ( gusset?)from the base about 1 inch and the engine'sbackplate is snugged up tight to that. it looks like it was made to have room for the old back carbed super tiger type engines