How to refer to the original object from which a model is created

Prototype has the exact meaning you are looking for, although you must be careful because in common usage it also means an experimental version upon which a more polished version can be based.

If you can clear up the difference using context, then prototype is fine:

The HMS Victory is the prototype for this scale model.

vs

I wasn't sure the mechanism would work, so I made a prototype from scrap wood first.

You could also use archetype, precedent, blueprint or simply original.

Do look up all of these words in the dictionary, for the fine detail of their various meanings; there are subtle differences.

If you are specifically addressing people from the field of Scale Modeling, who are expected to be familiar with the special import of the word, you may use prototype.

For the general public, you would better refer to it as the original.

As Peter says, I think "original" is the best word in general for this idea.

In one of those curious twists of the English language, "model" is another commonly-used term. Thus one might say, "Sally Jones was the model for this sculpture" or "I used the HMS Victory as the model for a toy boat that I made." But then we have the catch that "model" is also used to mean the copy! It would be perfectly accurate to say, "I used HMS Victory as the model for my scale model" -- perfectly accurate but very confusing! And then if you tried to explain differences between the original and the replica, would you say, "The model was made of wood but the model is made of plastic", etc?

There are several words which would do.

According to Wordnet, original, archetype, and pilot can all be used in the sense of something that serves as a model or a basis for making copies. Pilot won't work; it's normally used when a small scale model is created before the full scale vehicle, but you're interested in the opposite circumstance.

Google Translate offers model, example, and paragon as translations of the German word Vorbild you asked about in your comment. Paragon means a perfect example of something, and you are not looking for a word with that limited sense.

Here are some sentences about model trains which use original, archetype, model, and example to mean something that serves as a model or a basis for making copies. Note that model can mean either the the original or the copy.

• Pictured is a 1:8 scale replica of the original locomotive designed by Matthias N. Forney.
• The archetype for today's modern model trains is the Birmingham Dribbler.
• The model for this electric train is the "Train Eclair de luxe": the first Orient Express.
• This HO gauge caboose is an example of the typical crew cabin of the 1970's.