I think I might be close to cracking the 426M conundrum. In terms of deliveries from Toulouse the 426M aircraft are nested between the 429M and 338M variants
Inspections of exterior photos of A6-EUE show an unique feature - this being a single window blank at the rear of the second main deck cabin, suggesting row 66 might be absent (with the subsequent loss of 3 seats). If true, the seat count tallies at 426M, in lieu of 429M. Is there an extra lav at the starboard rear of cabin 2?