It is actually a 710 style rather than the later 746 style phone. Part of the Plessey range of phones for private systems or export - the 'main' station of a manager/secretary setup giving similar arrangements to the GPO's Plan 105/107. See
https://britishtelephones.com/ericsson/n1906.htm for more about it.
Telephone Rentals bought most of their range of telephones and PAXes from Ericssons/Plessey and stuck their own labels on them.
Ian J
ex Plessey Communication Systems