Yeah looks like a Concrete style top mounting style arm. There's a good number of a similar style as this in some parts of Illinois (mostly Chicago land area) and in Boston...except the arms are straight instead of curved, but I have seen other styles similar...like a davit arm style that mounts on top of concrete poles and a few others to name!

Also the "arm style" is similar to the common aluminum pole and arms used in Howard County near me! You can see one on the left side of this picture

I have seen these before but they are fairly rare around here.

I've also seen a double arm variant of this setup too.

I saw some intersecting arms in a side neighborhood (Looked JUST like the ones I saw in Boston) around the Boulder area.. I was on Highway 36 just leaving Boulder when I saw them.

The lamppost itself looks slightly like the european lamppost, except that in the european lamppost, the arm and the post is a one unit, and in this lamppost, the arm and the post is seperate units (Lamppost made from concrete, arm made from steel).

Post pole around here are concrete with metal arms, thats not uncommon but the arm in my area is.

Yep concrete poles with aluminium arms are commonly used here but the arms are usually bolted or clamped on the side.

Never saw a pole with a permanently attached arm, which is pretty hard to ship imo.