The definite article application

Could anyone dispel my doubts when it comes to the following issue. I was taught the definite article is necessary to apply if one refers to a known object. My problem is how to define a known object. For example if we refer to specific human being, animal, piece of furniture, vehicle etc, then is no issue. On the other hand, we may have a need to refer a particular type of an object. For instance, a kind of an electronic device, tool, container, car, which may be identified by a specific symbol. Does it mean that in such case we ought to use 'the' or omit it? I mean a sentence when we refer to a specific type, regardless how many identical objects we may think of. In such case we bear in mind not a specific single object but one of many others but of the same attributes. After all, in such case we will not treat them as a whole class, but one of it referred in a specific circumstances and it does not matter which particular object of a class is taken into consideration.

For example (it does not matter which one of the M6 bolts we mean):

1) 'Please apply the M6 bolt to fasten it'

2) 'Please apply a M6 bolt to fasten it'

3) 'Please apply M6 bolt to fasten it'

Which of the sentences are correct in the context of my concern?