If it's a 4 corner idle carb the back can uncover the idle slot also. You want the slots to be square give or take if it's a little more rectangle it's not the end of the world as long as your still on the idle circuit.
It is a 4 corner, it is a 650 cfm Speed Demon. Everything I see (and talking with Holley tech support today) they say the secondary idle slots should be invisible from the bottom. I'll probably just have to try it.