Maybe you are right. I just looked and it looks like the previous owner milled a new port under what would be the exhaust on the rear, and modified the front one to match the side bypass port. It goes pretty good that waythough. I haven't checked as a rear exh. because it may wear differently. Do you know how the MVVS .15 is supposed to go? The ports match up for a rear exh. but it came with a side exh. muffler. It runs the same rpm either way, even with a fairly bad mismatch.