#9178 has engine # 7216, written 'star 07216 star'. That's not too far out of wack with you, my chassis 25 units before yours and my engine 24.
I think George's rendition of things being haphazard at the factory is the most likely explanation, after all the first GT5 (show car) was reputedly #9250 and I suspect on the basis of orders from that show a number of coaches already at the factory were hurriedly converted to GT5. Coz told me #9178 was originally destined to be a GTS, but was converetd to full GT5 spec when an order came in for such.
I'm also second (or is it third now?) the opinion the 4 digit engine # is simply a typo.