Gear Oil-easy access

I was thinking that I would have to remove a bolt to change add gear oil, but there was a nice threaded cap just about like the engine oil only with no dipstick. I did have to remove a threaded metal bolt with a copper washer to drain it. The black rubber tube is a overflow. I put the top cap back on and blew into the overflow. That forced out the gear oil which was still fairly cool and draining slowly. I don't know yet what the clear plastic tube that is capped is for.