How do you check coolant on Cat D8n, newby question

Hi folks, new to the forum here. Looks like there is lots of knowledge here! I was told id be running an older cat D8N high track dozer with an open cab next week in a waste pit. Ive never ran one so I figured I'd take a sneak peak. Everything seem pretty basic except I can't seem to get the coolant cap off! It's a heavy metal cap with a tab that lifts up? When the tab is down the cap will not spin, when I lift it straight up it spins and spins but doesn't seem to unthread. Im new to this so I know I must be missing something. Probly a dumb question but once again I'm a newby here, thanks for any insight!

How do you check coolant on Cat D8n, newby question