Looks like aluminum tubes bonded with steel lugs and stays. Raleigh made the most bikes constructed like this with their Technium models. Reflex was another company that used this method of constructing a bike. Both of them tended to use shorter lugs, and I don't recall seeing the diamond cutouts on any other the Techniums or Reflexes I've come across. It still could be possible that one of them made it, but I'd suspect some other company is more likely.

Bonded frames do have a bit of a reputation for coming apart. Before you get too excited about this frame, I'd give a firm tug on all of the bonded joints to see if any indicate that they are willing to come apart. If so, better to find a different frame.