The "f" signifies that it was repaired and is now suitable for "field use" ie back into the war. There are a couple of possibilities with this piece....if it was made in 1913 then originally the fittings were brass and it was converted to M15 grey fittings. Look at the wappen grommets are they steel, any extra holes, any extra space around the grommets? The reason I ask, is they would usually pop the brass grommets and install steel ones as part of the conversion. The other alternative is repair of a hole in the shell or replaced/restitched visors. Look for any patched hole on the outside or inside of the shell. Second, pull the liner fingers outward and look at the visor stitch line inside, an extra line of stitch holes would indicate a restitch. Look at the stitch lines on the outside of the visors, sometimes they restitched using heavy white cotton thread and stitched every other hole. In this case, the stitches are longer than normal. All hauben have 2 parallel stitch lines, the first (closest to the rim) is for the liner/sweatband, the second is to hold the visors on to the shell. The liner is tack stitched around the rim of the shell, then the visors are stitched on front and back.
