In a booklet called "Rozeninzicht" published by DvP-CLO (Departement voor Plantengenetica en -veredeling - Centrum voor Landboukundig Onderzoek ~ connected to Rijkstation voor Sierplantenteelt Melle, don't know the specifics) I found this information about Melflor (so direct information of the breeders themselves):

Melflor: bred in 1982 from
"tetraploid R. rugosa seedling" x seedling

It was introduced by RVS Rijksstation voor Sierplantenteelt in 1987 as Melflor.

Further in the booklet, in the information of Melrose, a descendant of Melflor, it is mentioned that they've used a chromosome-doubled seedling of R. rugosa to create Melflor. This seedling was - very likely (mentioned elsewhere in this booklet) - created in vitro culture (so in a highly controled lab environment) treated with a colchicine solution to induce polyploidisation. So to create a tetraploid from a diploid parent.