@etch 15 February 1971 was just the official first day of decimalisation. A lot of businesses had been dual pricing for up to six months before. 50p was a good case in point as 50p coins were in circulation for over a year before decimalisation day, as the ten shilling note had been withdrawn from circulation.
This is definitely December 1971 - The Supremes played the Odeon on November 18th, and a second night was arranged for Dec 5th due to demand.
I remember I bought a signed copy of their latest LP at the time (Touch) on the night at the Odeon and still have it.
I've still got my ticket stubs for both nights so I will dig them out and get them scanned.