Just a couple of days ago I saw something like those on a 40’ trawler’s mooring lines. There were two on each line about 30 cm between them, on the two bow lines and two stern lines, but I did not notice the springs. I thought it looked interesting but did not see anyone to ask. I don’t see how they could provide anywhere near the shock protection, even with more than 2 per line. So, somebody is doing it.