SG37 - Die II Plate 56 1857
Only the plate number 56 is show in the left-hand corner, there is no current number as it is only first engraved on plate 98 in 1865
Image source British Postal Museum from the R M Phillips Collection. Vol XXII Pg 43.
On all the Perkins Plates the plate number is engraved in all four corners, the current number (from plate 98 only) is only shown twice above stamps AA and AB see here and below stamps TK and TL see here.