For those interested in the coinage of the Commonwealth this reprint from Caesaromagvs, the journal of the Essex Numismatic Society, should prove interesting; it provides an overview of Richard Cromwell.
Tumbledown Dick or Richard IV? – The Story Behind the Coin – Barry Lord
BNS Research Blog 
This article implies that the anchor mintmark on Commonwealth coins can be attributed exclusively to Richard Cromwell’s Protectorate. Dates given in the Sylloge of Coins of the British Isles volume 57 (page 20) state that the anchor mark was in use from 3rd December 1657, some months before Oliver Cromwell’s death. If this date is correct then the anchor mark was used under both Oliver and Richard.