To be frank, I think 'Rebecca Soper' is Europe's 'Miranda', and there'll never be a definite answer.
According to CP Photo Finder, it's [gracillima x ventricosa] - bred by Matthew Soper and being mass-produced in Holland. There's plants which show a strong ramispina influence, and others which definitely don't show anything like it...
On another forum, the general consensus was that gracillima is the parent in the cross.
On yet another forum, regarding this hybrid, I came across:
'Rebecca Soper' is definitely ramispina x ventricosa.
Matthew Soper told me he used ramispina for this cross but, at the time, ramispina was considered a synonym of gracillima (which is now called alba, gracillima is something else...;-)) Everyone there was in agreement that it's ramispina.
Wikipedia, which lists alba as a separate species as it is now considered, acknowledges that it was once synonymous to gracillima (their description both sounds very similar, too). There's also this:
http://www.necps.org/documents/Nepenthes alba and Nepenthes gracillima.pdf
Wikipedia also lists gracillima and ramispina as different species, and notes that ramispina is very similar to gracillma and there is much debate about whether the two should be treated as separate species.