The baseplate number suggests an
ATM phone:
http://www.britishtelephones.com/atm/l11560.htm
Very similar (chassis excepted) to my recent-ish charity shop purchase described here, which incidentally, is now our main house phone:
https://www.vintage-radio.net/forum/...d.php?p=992414
Is the yellow thing the cap? If so, how is it mounted?
Nick.