That sure looks to be about a 12-13 x 3" hex head bolt to me, or close. I work for a major player (the Blue one) in the nut/bolt industry and I would not consider that to be a "really heavy bolt", although it may be to you. Consider that I recently ordered twelve (12) three inch (3") x twelve inch (12) hex head bolts, that's 3-4x 12" grade 8 for a local industry. Oh yeah, the nuts, flat washer, and lock washers also. I'm still doubting, especially after the photo, that this bolt is hard, or even tough. Being as it is smaller than I had originally thought I am sure you can put the slot in it with the method I mentioned. At this time I am currently working on a quote to supply six (6) "tie bolts" for this industry for one of their machines which will be eight feet (8 ft.) long x eight inches (8") diameter and threaded with a twelve thread pitch on each end. That's one bolt. IF I get this bid then we will supply them for all six (6) of their paper machines. Maybe enough commission for a vacation?