Hey snik I'm still curious to help others. I do have the side fill "plug" that I can see like in the original OP's photo BUT it looks like a PO tried to knock it loose with a chisel putting gouges in it at some point, failing. I'm not sure there's any way to get the side fill plug in your last photo off of there. I feel like I'd heard or seen that this may have been just a cap you can pop off but I tried a little and it didn't seem like it was... There are no indentations in my side cap to insert a tool to loosen it.