What is the correct IATA airplane designator for the Boeing 787-10? Wikipedia claims 781, most other sources say 78J.  I can't find anything authoritative, and IATA has the gall to want US$578 for the answer.  Which one is it?

Update for clarity: Everybody agrees that the four-letter ICAO code is B78X. I'm interested in the three-letter IATA code.
I looked it up in the corresponding IATA document and I can confirm it is 781.

Sadly I think I am not allowed to attach any kind of proof from the document but the source of my information is IATA document 8803 - Aicraft Types in version no. 28.