Individual Members of a Group
When the individuals of any group are referred to, the non-specific article a is often used:
e.g.

S/he is a German (nationality) / Moslem (religion) / Tory (political) / Rotarian (association) etc.

When referring to individual people by their group, the same rules that govern common nouns--especially SPECIFICITY and PREVIOUS MENTION--apply:
e.g.

The Moslem from Kobe gave an interesting presentation at the theological conference.

There were two foreign passengers on the ferry, a German and an Indian. The German didn't speak any Japanese or English, but the Indian could speak both languages well.

There are a few other proper nouns that can be referred to with the article a. For example, days of the week can be preceded by a but the meaning changes.

e.g. If we say:

She will come on Monday,

it means she will come on the following Monday. However, if you say:

She will come on a Monday.

it means she will come sometime in the future on some Monday, but which particular Monday has not been decided yet. The can also be used to refer to a specific day of the week, future or past:

e.g.

She will come on the second Monday of next month.

I saw Mr Nakagawa on the third Tuesday in January.

We had a staff meeting on the Wednesday before last.

There is a flea market at Suma Dera on the fourth Sunday of every month.

We are going to Tokyo on the Saturday after next.

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The rules that apply to a common noun also apply here:
e.g.

Someone left an old Nissan near my house. After the car had been there a week, I called the police and reported it. (previous mention)

I know the owner of the blue Ford. (specificity)

That Mustang belongs to a movie director who lives in Kobe. (omit the article when another determiner is used)

Bill's Volkswagen is really old but it still runs well. (omit the article when a singular proper possessive noun is used)

Some drinks are also referred to by their brand names:
e.g.
Would you like a Guinness?

but also by their type:
e.g.
I'd like a good single malt whisky please. A Glenfiddich or a Laphroaig would be nice.

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Of course, the rules of SPECIFICITY or PREVIOUS MENTION still apply:
e.g.

Among the works in the exhibition were a Dali, a Higashiyama and a Warhol.
The Higashiyama was the only Japanese piece on exhibit. (previous mention)

Some unique musical instruments, pieces of furniture or other art objects are also referred to by their maker's name:
e.g.

That violin is a Stradivarius.
That chair is a Chippendale.
That katana is a Masamune.

However, periods of time in which works of art were produced are usually preceded by the:
e.g.

The Renaissance / The Edo period / The Baroque period / The Heian period
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John: 'Did anyone call when I was out of the office?'

Bill: 'Mr Ono and a Mr Tanaka called this morning.'

John: 'I'll return Mr Ono's call. What did this Mr Tanaka want?'

Bill: 'He said his company makes educational software.'

John: 'We don't need any of that. We can make it ourselves!'

Bill & John both know Mr Ono but neither of them know Mr Tanaka. They don't even know his personal name and Tanaka is a very common family name.

The article a is used before a person's name if we are unfamiliar with the person :

-- 'a Mr Tanaka called'.

In John's reply, he said, 'What did this Mr Tanaka want?'

Using the determiner this before a person's name indicates unimportance to the speaker.

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