I believe GPO does indeed stand for General Post Office and it seems to appear on a variety of tools in the UK. I know very little about the Red or Blue Diamond series, but my impression is that they date to the 1960's and were perhaps produced through the 1970's. I really like the short Wren...
This is my M&W "Scrutite". It's 11" overall and quite heavy. It shows up in my 1955 cat. and probably later. From the description in my 1939 cat., it looks like it may have just been introduced then or soon before. It's a nice screwdriver.
Gerry
My guess would be Britool. Mine is flimsy, with the minimum of rivets, but I have seen them with four rivets in the head although not in that exact placement.
Gerry
They do look very nice. Is that baked oil a durable finish? I'd be curious to see if there's a simple, safe way to japan black or parkerize in the home workshop with readily available chemicals.
Gerry