The 482102 is the earlier version of the rail joining bars (you need 2 of them) and are what I use on the very, very rare occasions that I have to join 2 rails together. I bought mine back in 2015.
The 577039 are the latest much improved version and come as a pair ( about £40).
Quite honestly, if I had a need to replace mine for some reason, I would go with the newer version and pay the extra £8.10p
The latest version is self aligning, whereas the early version requires an accurate straight edge to ensure the joined rails are perfectly straight. Even after the rails are joined using the early version, it's really easy to get the joint to move and cause inaccuracies whilst moving the joined rails around.