Shaw,
Welcome to QB forum.
The B.R.7 number is the motor number. Your machine is probably a Model 127 with the Sphinx decals by the looks of it.
It is an electrified treadle machine as the cabinet is a treadle cabinet with the big wheel, foot pedal, and associated parts removed.
The serial number on the machine is stamped on the machined area on the bed directly in front of the vertical pillar.
With the entire serial number, letters and numbers, I doubt you'll find it's as old as you think.
Joe