I suggest this rose is a sibling of Alister Clark's 'Lady Huntingfield'. "Dettmann No 6" has obvious Pernetiana parentage. So, when you check its parents, has 'Lady Huntingfield'. Scores of Clark seedlings were in the Dettmann's garden for trialing; this may be one of them.
The main grounds for siblinghood at this stage are striking visual similarities. Below from left, Dettmann bud opening, Huntingfield bud opening, Detmann flower wide open, Huntingfield flower wide open.
Se when we're checking parents we should be looking to 'Busybody' and 'Aspirant Marcel Rouyer' rather than 'Souvenir de Gustave Prat'.