This is an electroplate mark of Favell, Elliot & Co of London and Sheffield. The “L” here is for the London branch of the company. RF is Rupert Favell who was in partnership with Henry Elliot. “1991” is most likely an internal factory design number specific to your cup. The partnership started in 1883 and ceased in 1893 when the business was taken over by another company so your cup must date from that period.