From the 1955 service manual:

The "Specialist" TK 820 is a portable Tape Recorder capable of recording on 1/4" magnetic tape.  The machine is a dual track recorder and is able to record and play back in both directions.  It operates at the two tape speeds of  3¾ ips and 7½ ips.  The Electro-Magnetic change over from track one to track 2 position takes less than one second at 3¾ ips and four seconds at 7½ ips.

Drive is obtained from a split phase induction motor. Input selection selection is made by means of a three-position switch.

Two recording and two erase heads were fitted, offset for the respective upper and lower track positions.

Three loudspeakers were incorporated into the cabinet of the TK820 giving a wide range distribution of all audio frequencies.

There are additional images and a belt configuration diagram for this recorder on the CD-ROM.