RSR in the exergue of Carausius’ coins represents lines from Virgil rather than a mintmark.
An interesting and detailed review of Carausian coinage by Ken Elks is here.
He notes: (text copyright Ken Elks 2015)
R.A.G. Carson, in an article published in the Journal of the British Archaeological Association 1959, “The Mints and Coinage of Carausius and Allectus”, further embellished in another article, part of “Mints, Dies and Currency” published in 1971, proposed that these coins, both the RSR series and others without mintmark, were minted at Gesoriacum, pointing to the fact that their distribution in British finds is almost entirely confined to the south-east of the country and grows as a proportion of the coins discovered the nearer the finds to the coast. In his view, support for this theory came from the fact that this coinage ceases after AD 293 and around the time that Gesoriacum was lost to Constantius. This view has since been challenged, not the least because the RSR coins have now been found die-linked with coins from a mint in London. Explanation of the RSR mark is not obvious and various suggestions have been made, including Rationalis Summae Rei (Aurelius Victor refers to Allectus as “cum eius permissu summi rei” ) or Rationalis Summarum Rationum (the title of an officer in charge of the mint). However, by far the most plausible (and now widely accepted) is that recently advanced by Guy de la Bedoyere who believes that it is a quote from Virgil’s Fourth Eclogue, “Redeunt Saturna Regna” (“The Saturnian Age returns” – in other words a new Golden Age). If this seems far-fetched, he also points out that the letters in the exergue of the medal of Carausius shown above, I.N.P.C.D.A. are the initial letters of the very next line, “Iam Nova Progenies Caelo Demittitur Alto” (“Now a new generation is let down from heaven above”). Such sentiments would be totally in keeping with the aims of Carausius expressed elsewhere on the coins, for example ROMANO RENOV(at) (“Rome Renewed”).
Roma Numismatics > When Carausius settled in Britain in 286 the Roman currency was in a degenerate state, made up almost exclusively of base-metal issues; he saw an opportunity to use the platform of coinage as a means to present himself, his regime and his new ideology for the breakaway ‘British Empire’, and gold and silver issues superior to those made by the legitimate empire were the principal manifestation of his traditional standards and virtues.