It looks to me like you are already down to the tank. Put a strap or chain wrench on the fitting and see if it unscrews. If not, you could try to pry it out. Once you have it out, you can identify what is was and replace it.

If the access isn't required by local code, you could bust the thing out of there and just put a cover over the hole. Many people have strong, but conflicting opinions on how often a septic tank should be pumped. I am not sure who to believe.