Frank, unfortunately, someone saw the post and has claimed it. If it falls through, I'll let you know.
In answering your question though...yes, it's a hole. For what, I have no idea. Done by previous owner. But it shouldn't affect anything. And I'm located in Queens, NY...11355. I think the sump was slightly wider than 12", otherwise, it was your fit.
Check out Jung's (gforce)...great guy to work with. And took a glimpse of the sump...great condition.
I can make you whatever you want...as simple or complex as needed...I'm finishing mine up right now...about 55 gallons total volume, I'll also have a modified CPR sump available in about a month...I think it has a total volume of around 25 gallons...
Frank you still need a sump - I had one at the frag swap with all my frags in it -
30g exact dimensions you outline and it has 4 baffles, 3 inlets - 2 for drain and 1 for top-off and it is drilled for 2 returns if you want, or you can use a mag drive like I did and not use the bulkheads on the return.