Hi Barnaby, they are the Bachrus rollers, and I've used these for N/OO/EM/p4. Quality is good and for the 4mm scales if you have chaired track and aren't careful they can sit so they aren't square, which can give poor running with the appearance of quartering being incorrect. On the likes of Peco they work without any issues as the roller mounts sit square on the rail surface. There is an Allen key/grub screw to hold the gauge, again if this is too tight it can pinch wheelsets giving poor results. I'm very happy with mine, they've given a good few years reliable test and running in useage.