It was made, quite literally, with a coachbolt. There happened, at the time, to be one handy. The round top of the bolt was pressed into the blob of glass put over the pontil scar. It leaves a round outside ridge with a round crater inside. The coachbolt pontil mark.
(the pontil is the metal rod the glass is held on while being worked, when it is broken off, it leaves a MARK. The pontil mark or pontil scar.)
(nag, nag, nag!) :wsh: