Are you pushing the pack into the camera at that same point every time? Kinda looks more like a pressure mark than a light leak (though it's hard to tell it's so tiny). Otherwise it might be a pin hole in the bellows?
Since images are upside down and flipped left to right, I'd use that information to find the pin hole... maybe top left area?
Also since it's a conversion... have you checked to see if the lens/bellows when closed up press against the film pack? That might cause a pressure mark.