M_Boats: Cracks in locker

Hi I just bought my first monty 15 a 1985 hull # 321 and after cleaning out all the old stuff in the port seat locker I found cracks . They are where the plywood bulkheads and the hull meet , what I'm trying to figure out is .Is this some kind leveling compound or seam filler , and if so what do I repair it with Thanks KLaas