Identifying mod to 410/130HD

Hi, I have a 410HD that was modded by a previous owner. Looking at the amp from the back, the are two unlabeled jacks on the far right, and the wood there has been cut to accomodate them, so it's clearly a mod. Can anyone guess what these do or how I might test them? I thought it was an effects loop, but when I ran it through a pedal the resulting signal was very faint. I can post a photo if that would help ID, but I'm not sure it would.

did you try running the effects both ways ? also one could be a line out, just a guess.