Oh, I totally agree it seems odd to forge such a low value coin. Even adjusting for inflation, calculators reckon it'd be like forging ~20p coins today. I.e. too small to be worthwhile.
The detail does look lacking to me: folds in Britannia's robes, the lines on the shield, lines in the plume of her helmet & on George's hair too. The real coin is fairly worn but you can still pick out some of those details with relative ease. But on the “fake” one, the far side of George's hair looks almost like a singular block.
Its like the biggest details have survived reasonably well but all the finest details have been lost.
It looks like someone took a real coin, made a mold of the front & back but messed up the depth. And the pressing they took didn't get the finer details. Possibly cast it in a different cheaper metal explaining the colour.(?)
Why? Who knows.
Could be a prototype as you suggest. Perhaps trying thinner coins because of scarcity during WWII. It's possible. But I maintain the forgery is more likely; perhaps more as a test of what they could create more than any desire to defraud someone. I.e. just some bored bloke in his shed messing around for a laugh.
You could drop the Royal Mint museum an email with some pics. If there's prototypes, they'll likely have some or at least paper records of the work.
