Maybe GB (Gerry Burgess)? Their top of the line spearpoint stem in 531 is stamped with their logo, but this might be one from lower in the price-point lineup.
Here's the 531:

The mystery stem could be someone else's knockoff of a GB spearpoint too.
They made the spearpoint in Hiduminium (Al alloy) too, later than the 531 I assume. With a fake lug shoreline that mimics the steel lug.