The shape of the punch is similar to the one shown by Grimwade 3755. There is a pellet between the initials there too but the letter style is different. He saw this mark on a fancy back spoon of around 1765 and gives a possible attribution to Philip Roker II but if his comments in the biography are correct and PR II died before 1757 that attribution seems wrong and, if a Roker stamp, it ought to be PR III.
In the biography for Philip Roker III Grimwade suggests a mark was entered around 1758 which was in the lost register. What you may possibly have here is an example of that lost mark. The shape looks like Grimwade 3755 and the lettering and pellet are very much like Grimwade 2227 of 1776 which is attributed to PR III.