The photo on the 600 Squadron website does indeed show the Sovereign's Colour for the Royal Auxiliary Air Force (left) and the unofficial Squadron standard (right). See
RAF Reserves - 600 (City of London)
The latter standard is known as the Collett's Standard and was presented to 600 Squadron by Lady Collett, wife of the Lord Mayor of London in 1935. The Standard was in memory of her son, Sqn Ldr Stanley Beresford Collett, OC 600 Sqn, who had been killed in a flying accident the previous year.
Although 600 Squadron has only one, unofficial, standard, it is the only RAF squadron with two official badges.