The 40 horse (prop) and 30 horse (jet) are the same powerhead exactly, it's just the lower end that is different.
So mechanically on the engine end they are the same unit.
The Jet is only rated at 30 horse (at the prop end of things) because jets are only about 75% as efficient as a regular prop so a 40 horse prop converted to a jet will yield "about" the equivalent of 30 horses. When you look up parts for the 4 cylinder 40 (from that serial range), it will also list the "jet" components along with the normal prop stuff.
So unless the serial (production number) the owner provided is "bogus", then there is no concern.