These are made by Stanley. There was a little nut/bolt in the slot that sets the wire gauge, but I removed it and do it by feel. These date back from the 70's.
now being toothless/fingerless and having burnt myself , I have ordered
the bt mans supertool at the horrendous price of £2 and another from china at 99p.
once received its a practise on spare bit of bt phone cable.