This appears to be the master link, but it's not split like the repair manual shows. I'm not sure how to separate it. The chains are Nexus which I assume are aftermarket. Don't know if they're lubricated or sealed. The pins are flat on one side and have an indent on the other. The previous owner replaced the chains, pads, sprockets, rollers and the left side idler (new design). The right side idler brackets (old design) have excessive clearance with the frame. There are no shims on either side, and the brackets have actually worn down on the mounting surface. The idler rim is also pretty thin. I was lucky to find a new aftermarket idler locally for a great price, so I decided to go that route. I just need to figure out how to break the chain. The pic shows the left side but the right side is the same.
I think my tracks have "gator links" as described by the honorable materthegreater. I did a search and found this pic he posted. If I look closely on mine I can just see the teeth and the splitline.
The gator link like you have is held together by the track pad bolts. Park the machine with the link on top of the idler, remove the bolts and track pad, and it should come apart easily. You may need to use a small hammer or pry bar to lift the link enough for the teeth to cover each other.
I can see it if I look close. Just clean the paint off and you'll see the serration in the links more clearly. They wouldn't make a solid link. Loosen the track before taking it apart so it's easier to put back together.
Look after those bolts. In Australia it's easier to find rocking-horse poo than get new long track bolts.
Broken Tractor has sets of 4 bolts and the square nuts for $5 USD for the 450 and 550 models.
No nuts are required for the split link, the bottom section is threaded.
The two links slide together. You will need planty of slack to get the chain apart. Spray link generously with a good penetrating oil. Place a sledge on the top of the one link and use another sledge to strike the bottom of the other link. There was a tool to get these apart. However this does the trick too.
Another tip when reassembling, Things need to go back nicely.No pulling it all together with the bolts.Should be almost hand tight.