Mock string lines were applied by roller to registered letters at Liverpool in lieu of green string, which had been used until December 1877. Different rollers have been recorded used from 1878 until 1881 with the string lines inclined either to the left or the right, like this example here.
This registered cover shows 2x 1d. plus 2x ½d. paying 2d. registration fee plus 1d. postage. The stamps have been cancelled with a vertical oval “466” handstamp of Liverpool post office. The blue oval “Registered” postmark struck on the front may indicate that EXCHANGE B.O LIVERPOOL was the office where the roller string lines were applied.
Back stamped on arrival in London later the same day.
Find out more here.