Hey Eugene -
Through my study of the scriptures I have found if the number 6 has any symbolical or spiritual meaning it probably should identify with work. The number 6 signifies a theme of man's work illustrated throughout scripture:
God worked 6 days to create the universe, man was created on the 6th day, men are appointed 6 days to labor, a Hebrew slave was to serve 6 years and be released on the 7th, 6 years were appointed for the land to be sown and harvested, etc. And the number 666 of Revelation 13:18 identifies with the unbeliever non elect who essentially are working to get right with God.
You are correct in saying that the number is not a number of "a" man, but the number of man. Just like we don't translate Christ as the Son of a Man... He was the Son of Man. He was of mankind. In other words He was a human being.
And here also in Rev. the same word is used, so it would have to mean that it is the number of man, the number of mankind, the number of the human race. Anyone who is an unbeliever symbolically has the number 666 on his forehead. It is assigned to the non elect who is destined to the lake of fire after the first resurrection. To make this more apparent of mankind working for their salvation, here are a couple of examples:
In Zechariah 13 in verse 8: Two-thirds I will cut off and destroy, but one-third..." and then it goes on to speak about the redemption of the one-third. The number two-thirds, if written as a decimal, is .666666, and if we just use the first three numbers, we have 666. We read about David, when he defeated the Moabites, that he caused them to lie down in three lines. Two of the lines he put to death. One line he spared, and they served him. David there was a type of Christ, the two-thirds a figure of those who are under the judgment of God and the one-third a figure of those who are the elect. In II Kings I we read of Elijah, that he was sent for by the wicked king of Israel. And a company of men came to take him and he called down fire from heaven, and they were destroyed. A second company of men came to take him, and he called down fire from heaven and they were destroyed. A third company of men came to take him, and plead for mercy, and they were spared. Again we have two-thirds being destroyed, and one-third being spared.
Now with what I have shared... I wouldn't take it as "thus saith the Lord" but what I have observed, and so it could possibly change with newer understanding with others comments.
doug