I've got one of each, (stamped and unstamped) so definitely not always. It isn't always easy to see, depending on the enthusiasm of the man with the hammer.

We bought a Honda farm bike years ago. usec oil from day one. Pulled it down. Missing a scraper ring from the factory. That's peculiar. For a Honda anyway.

The "Man with a hammer' comment was perhaps a little tongue in cheek. The larger stampings I've seen, like the cross members, firewalls etc, seem to be a little too consistent in depth to be done with a hand punch set. They are more likely to be done in some kind of press. Your double stamped item might be a double hit in the press on the way in or out.

Gerard, the letters and numbers probably are the date stamp, as it sounds like its in the right place for one. I'm not sure if you know how it works, but the first one or two numbers are the day of the month, followed by a letter, which is the month, followed by a number which will be the year. For the month, the letter "I" is not used the avoid confusion with the number 1, so the letter M will be the month of December.

For example, 5J9 would be the 5th of September 1969, and 16M0 would be the 16th of December 1970. It's also common to find other letters after the year numeral, although I'm not sure of their significance.

You will find a similar system used on castings, and also often a clock face to indicate the time of day of the casting.
Castings of American origin also reverse the day of the month and the month, so 5J9 would become J59, for example.