Could this also be ' This is the Saturday pencil'??
No sorry. That doesn't make sense in everyday speech. Not unless it's some sort of inside joke.
I could see it. If I use a different pencil every day of the week, this one here is the Saturday pencil.
That was also my first thought, but seventh makes more sense.
...cua con trai thu bay...
Couldn't it be: Here is the seventh pencil?