Yes, it's Aluminium. It's I think from a US made enlarger, not anything I've seen in the UK. US enlargers were usually brought by Americans sent by their companies to work here (or military personnel), or the opposite Brits returning from working in the States.
The Beseler 45MXT lens boards are I think 4" square. I know I photographed it, but when - from memory it came in a darkroom job lot around 1999, however I photographed it more recently, along with a baby Linhof lens board.