Lots of history. It was issued as part of England's switch from a pound made of twenty shillings each worth twelve pence to a pound made of 100 pence (Decimalisation). That's why it's 50 New Pence. Latter issues dropped the "new" part.
This was the world's first heptagonal coin. In fact, the edges are slightly curved ... what this means is that the diagonal across the coin in any direction is the same, so that it will 'roll' like a circular coin, and will therefore work well in vending machines. The technical term is an "equilateral curved heptagon". As Kani says, D-Day was 15 Feb 1971, but the new 5p & 10p coins were issued in 1968, and the 50p in 1969, with the bronze ½p, 1p and 2p in 1971. Not worth anything more than face in this grade, a few £ in Unc. The 1970 issue had a mintage of 19m, so is worth a bit more. The "NEW" was replaced by "FIFTY" in 1982. It has been minted in proof sets from 1971 to today, and for circulation as follows: 1969, 70, 73 (special EEC commemortaive), 1976, 77, 78, 79, 80, 81, 82, 83, 85 (just 605,000), 94. From 1997, its size was reduced, but it remains the same shape; released every year since then. 1982 saw the first of the annual Unc sets, so the 50p can be found in Unc for all years from this date forward, but for many, it wasn't issued for general circulation. The reason it declined after 1982/3 was the introduction of the 20p, which meant the 50p wasn't needed as much. Does that help?
