The Balun cores used were initially obtained from Rockby Electronics and were subsequently found to be equivalent or equal to a type 43 size 202 core. The Jaycar ones are not quite the same material though they are the same size, the type 43 cores will work better at lower frequencies the Jaycar type M7 may be better at higher frequencies, though a type 61 may be better still. For my latest RLBs I use a combination of 43s and 61s, with the 61s closest to the resistor end. I have also found that the thinner RG178 (available from MiniKits) type coax is easier to work with.
1. A Simple Wideband Return Loss Bridge Revisited by Paul McMahon VK3DIP - Amateur Radio Magazine June 2007
2. Further Reflections on a wideband return loss bridge by Paul McMahon VK3DIP - Amateur Radio Magazine August 2008