They trialed 20 rounders for the No 1 MK III in world war one they worked better with 15 rounds in them they were not a great success as being extended and cumbersome they caught on stuff added more weight to the rifle and were disliked by the troops if you can find one then they would be about as expensive as a Hiscock .22 magazine which will set you back @$1500-1600/Au.

As for the No.4 or 5 rifles having a 20 round mag never happened.