NESTOR

Some definitions:
£sd = pounds, shillings, pence
£1 = 4 crowns = 20 shillings = 120 pence
1 guinea = £1 + 1s
2s coin = florin

Stephen's pay of 'three twelve' means 3 pounds and 12 shillings. It can be written as £3-12-0 or £3/12/0.
Mr Deasy gives it as: 2 £1 notes + 1 sovereign + 2 crowns (= 10s) + 2s.

The next step is understanding this postcard!