Looks like I made a schoolboy error when it came to measuring the bike :facepalm:
It's a 55cm, not a 57cm which explains the frame number better, and also why the frame is a bit short (I can fit a longer stem though as I've only got a 90mm on there). The frame would have been made by TiSport and is either the 84th 55cm Haole frame that they made, or the 84th 55cm frame of any brand made. That part may well remain a mystery!