Thanks for the information. Boeing Customer Codes is exactly what I was searching for. I just did not know that the codes were sometimes used to describe certain aircraft models, e.g., Boeing 737-338.
Now it all makes sense (737-338 = Boeing 737-300, Qantas).
It is not really describing "certain aircraft models", it only tells you who was the customer who ordered this aircraft new from the factory. When aircraft changes airline and/or ownership, customers code remains the same, so 25 y.o. airframe which has changed 10 hands since new would still have "Qantas" customer code.
And also this has nothing to do with technical aspects. For example Luthansa's 737-330 from mid-80s delivery batch would have some technical differences from early-90s delivery batch, both still -330.