CW1724 has been discontinued by us, it was basically CAT5 spec as far as the internal twists go but is only 2 pair, we've switched to 4 pair CAT5e as it's easier to get and offers more future proofing if required. Re: The master socket, the Mk4 is VDSL, the other socket you have linked to is NOT designed to be used as a master socket (no technical reason why it won't work though) & was designed for ADSL. If you have extensions then the obvious choice is the Mk4. W can't compete with the eBay prices, we have to buy out stock!