As suggested by so many, I contacted RP7 and had a reply within 24 hours. The brand manager wrote a very nice email, confirming I received a Birkin35 insert. She suggested to refund the order. The refund was credited within minutes.
She explained:
“To perfectly fit most Hermès Birkin bags, L'Insert B35 must be 31 cm long. That is […] the most suitable length to make L’Insert functional while keeping the matching Birkin 35 fully functional too”
Also she said:
"L'Insert isn't a liner […] When designing it, we didn't intend for it to line all inner sides of its matching handbag […] it was designed to use the inner available space in the most efficient way […] while keeping the bag functional, open or closed, full or empty."
These explanations make me understand better what the 7RP insert is about, what they can do and what they can’t.
The insert seems very good for organising. But since it is considerably shorter than the bag, in my view it
does not double as a liner and
does not protect reliably the interior of my bag. When I toss things into my bag, I don’t want to worry if they fall into the gap or not!
If only this would have been more clearly stated on the website the hassle and disappointment of this purchase could have been avoided. On 7RP’s site, the fit of the insert is described as "snug as a bug" and that it is “designed to fit your bag to absolute perfection”. From this, I didn’t expect to find it leaving a large gap in my bag. And especially not since 7RP says on their site that “L’Insert protects the inside of your bag from scratches, stains and liquid spills”. I prefer to have the entire base of my Birkin protected, and will look now for an insert which organises and protects.
A final remark..I looked again at the earlier pictures of 7RP B35 inserts shown in this thread, and (as the posters say) they look flush against the bags (snug fit). Maybe at some stage the 7RP B35 inserts were longer than the 31cm they are now? I am curious why they are shorter now. It certainly would help with the problem little.bear encountered with her 7RP K28 insert a few pages ago.