There are 2 holes drilled in the bottom of the tank. the return , and the supply. The overflow is threaded to the bulkhead and is a 1"tube that is almost the full height of the tank. Around the 1" tube there is a 2" tube that has a strainer at the bottom and at the top. The bottom looks like a typical strain on a filter. The strainer at the top has a cup shape that is open at the top. Inside that cup there is a cap/lid with a hole in it at the center. In the hole there is a piece of rigid 1/8" tube that goes down in the overflow. If the 1/8" tubing is raised or lowered the water level in the tank will lower or raise.
As I read this beck, before sending it types way more complicated than it looks