We had a TR 95 so definitely an older vintage but it got me curious so I went into New Hollands online parts and I think I may have discovered a serial break where the last of the TR 86's didn't have the second tank.
Just as you said, our TR 95 had a hydro tank on the left rear of the grain tank just in front of the engine. Then it also had the hydraulic tank very close to the rad to the right of the engine.
If the link works I pasted in, follow along on the pages from 1 through 8 and note the photo grouping item number on the left side. On page 1 go to items (028), (029), (030) and open each one and you will note on 028 and 029, they show a reservoir tank { they name it a brake fluid reservoir of all things, no wonder its hard to find ... goofs ! } . But then in item 030 and on page 2 in item 031 they no longer show that tank. To me that probably means they did away with that tank although this is only speculation on my part. Match up your serial number to that listing and it may be a light bulb moment.
Then go to page 4 and items (107), (108) and in both items they show the same tank but a serial break there as well and some plumbing differences and I expect that is the tank you are staring at.
As a hint to see things better you can blow up the page to full size with the green key in the upper right hand area of that screen, then also you can click on the zoom button at the bottom of each picture and zoom in and also hold your mouse down and move the photo around to see things closer.
If you haven't been looking at this online, save the NH site so you can reference it in the future although as I have found out with NH and Case IH, this online system is not the same as what they use so pages and so on may not be the same and part numbers could be changed up, they sure know how to make a mess out of things that could be easy !.
Official New Holland Online Parts Store and Online Parts Catalog for New Holland and Ford Tractors.