Being English, I can answer this:
Yes it is a tax disc. In England they were/are renewed annually and had to be displayed on the left of the bike.
They made it so you had to show evidence of insurance and ownership to get yours updated. That way the police knew you had those things. I could not afford insurance a lot of the time when I was a kid, so used to forge my tax and put on the bottom of the left fork leg pointing downwards, so mr. policeman had trouble seeing it, especially if I pulled up by a big puddle. Jerry