Interesting details Björn (and Tony)
First of all the He 42 production is confusing
It seems all sources agree with regard to the Early ones built, ie up to He 42 D.
But the He 42 E creates a problem.
Volker Koos states 189 were built
Björn above says 217 (241 minus 24, those built before the E model)
When counting all the known WNr I only get 175
With regard to the D-IVBY vs IV+BY I must say I doubt Luftwaffe did it that way. One possibillity would be if there was a remaining He 42 still using a WL code, ie WL-IVBY or WL+IVBY that late.
I know that the WL prefix was around much later than normally assumed.
Cheers
Stig