The Junkers R42 is the only aircraft type, which is known under the number sequence "42". However, these R-designators were just used for aircraft types, which were built or intended to be built in Fili, Russia.
They normally did not block a number within the designator sequence of Junkers.
Therefore it is not expected, that the R42 fits with the J42
The J42 designator might belong to a early stopped project of 1931