Some interesting pics of a Falke 80 here from John in the Netherlands, with the second digit of the serial number tilted 90 degrees. Like the rifle in New Zealand pictured here:
This latest development, when put with the other occurrence of this pattern, seems to support the idea that the numbering of the Falke underlevers is in fact a series of makers' marks - identifiers added by the craftsmen - because there is generally one stamp on the loading tap and another on the cylinder of each rifle - two items that need to be matched during the manufacturing process.
This would explain why duplicate and even triplicate 'serial' numbers etc have emerged: maybe there was no organised effort to keep a chronological numbering system and as long as no two rifles with the same number were on the production line at the same time, it wouldn't have led to confusion.
Another piece of evidence supporting this theory is that it appears on the break barrel Falke rifles no number or other individual makers' mark was stamped - because break barrels do not contain parts that have to go together, as opposed to the hand-honed breech blocks and their matching loading taps on the models 80 and 90 underlevers.
To sum up, it is possible the Falke underlevers never had a proper serial number at all, just makers' marks that were useful in avoiding confusion during manufacture!