Alex
Andrzej Morgala is a Polish author/researcher who has been writing books about the Polish Air Force since the 1970's.
I was quoting from a book published in 1976, Polskie Samoloty Wojskowe 1939-1945. Back in those days I assume it was not as easy for a Polish citizen to check RAF aircraft details as it is today. I also assume, as Wojtek already pointed out, that Morgala probably got a lot of details from log books, and he either misinterpreted the details he read (sloppy hand writing is something everyone fights with when researching handwritten notes/documents) or the details he noted down was wrong, for any number of reasons.
Unfortunately none of his works has been translated into English (as far as I know) which is a great pity, since no one has gone into such details with regard to serial numbers etc, especially his two books about the inter war period are magnificent, even for a non-Polish reader....
Cheers
Stig