Again, this is just educated guesswork, but obviously Methuen wouldn't have been reappropriating or claiming the French-language
publishing rights to Tintin books with this sticker, since those rights were already held by Casterman. Rather, I'd guess that this sticker indicates that Methuen had acquired the
sales and distribution contract for selling Casterman's French-language Tintin books within the UK market. This kind of deal would have made sense to both parties, since Casterman wouldn't have had any of their own sales reps in Britain to supply British book shops with their books.
The fact that the sticker has a Methuen catalogue number suggests that these Casterman French-language Tintin books were in that year's Methuen trade catalaogue, from which retailers would have been ordering stock.
MrCutts:
All I can say is that I've never seen a Methuen sticker in a Casterman book. It just seemed interesting and unusual. Perhaps not that unusual.
It's not unusual for publishers to handle another publisher's sales, even within the same country. For instance, there are quite a few small UK publishers who contract out their sales and distribution requirements to a bigger publisher that has its own in-house sales department. But I think you're right that it
is unusual to see such a sticker inside a book. Maybe it was necessary in this case so that bookshops and warehouses knew that these French-language comic books (which were presumably something of an oddity and being stocked in fairly small numbers) were to be treated as Methuen stock.