Rp is an abbreviation that is used by Mayerne in his manuscripts. In our reading it refers to recipe: a set of instruction for making something.
Rp is here understood as a synonym to the more common ℞. The use of the word recipe as an opening instruction of a description of a procedure to compound specified materials in a particular way with the intention of creating something. The Late Latin verb recipe, the second person singular imperative form of recipere, "to take", thus: "take thou". It was first used in medicine in 1519. From the 18th century onwards recipe was also used for cooking. The word receipt is derived from recipere, and comes into use as 'a written declaration of having received' from the 17th century onwards.
"recipe, n." OED Online. Oxford University Press, July 2018. Web. 6 September 2018.