Pattern maybe struck when a new coin metal or design is to be selected. Since only a few pattern coins are minted, and most are melted at mint the pattern types which are not selected for circulation become very rare collectors items found mostly in mint museum collections.
A extreamly rare Copper Proof 5 cent Pattern dated 1904 of Edward VII King and Emperor was struck with design of the 5 cent coin of Queen Victoria issued from 1870 to 1898. Considered too large it was not issued to circulation and was replaced with smaller Cupro-Nickel coin adopted by proclamation dated 1909 December 13th.
In 1942 new type of coin was investigated to conserve metal for the war, A One cent Pattern was struck in Black Bakelite plastic in high relief. I understand it was manufactured by a Private Firm and sent to Ceylon for evaluation. This was not viewed kindly by the Royal Mint which took action against the Firm This pattern was anyway not seen to be durable for circulation. 1942 September 8th, a bronze coin of half the original weight coin was selected for circulation.
In 1942 the Silver 10 and 50 Cents coin were Demonetized by proclamation of Governor dated 1942 July 30th with effect from 1942 September 30th. A rare Nickel-Brass Proof Pattern dated 1943 with design of the 50 cent silver coin was minted. It was not issued into circulation, and was replaced with a 50 cent coin with a new reverse design.