This is an old Jaybeam aerial, and Jaybeam (before they left the UK domestic aerial market having foolishly tried to sell top quality products) included a balun in all their aerials. A balun makes sure that the signal transfer from the aerial to the cable is highly efficient and properly matched. Then some dork comes along and shoves two cables into the box. Oh dear.
Many thanks to Bill Wright of Wrights
Aerials (Tel/Fax: 01709 813419 / 07860 610519) for the information and
for the comment above.