Case 580b what is this reservoir canister?

Hello everyone, new to the forum and would like to thank everyone in advance for the help i have already received just reading through these threads. I have recently scored a 74 580b and everything seems to work great. I would like to keep it that way so i have a complete fluid change on slate. I am getting familiar with the machine as i go. Most is self explanatory, but i found one reservoir just behind the alternator that looks like it is hydraulic oil. Is it some sort of external reservoir??? I have looked high and low in the service manuals i have i can’t find it anywhere. Here is a pic… any help would be greatly appreciated…