JonE, the "adjustable" or "telescoping" lower links are quite common on larger tractors, and becoming more common on the smaller ones apparently. I don't have a picture, but they have a release you press on the top and you can extend a lower link to hook up an implement, then when you get both hooked up and pick up the implement, they slide back in and lock in place. And now I wish I had looked more closely at that B2910 a few days ago to see whether the lower links on it and the ones on the B2710 are interchangeable or not.
Bird