The original NTSC version of the chip (AY-3-8500-1) arrived safely from the US. I connected it up and it works, so I was able to to an accurate side-by-side comparison between my implementation and the oritinal NTSC chip. I have now updated the code so that it accurately implements the NTSC version as well as the PAL versions.
Differences between the NTSC and PAL versions...
1. The scores on the NTSC version occupy 20 scanlines (4 lines per block) but the PAL version occupies 30 scanlines (6 lines per block) - as on the datasheets. The NTSC numbers are noticeably shorter.
2. The vertical net/centre line on the NTSC version is SOLID but the PAL version is DOTTED (as can be seen on the pictures below).
3. The NTSC picture has fewer scanlines, so a lower vertical resolution. This results in the ball appearing to travel at slightly steeper angles on the NTSC version because each step therefore has a larger Y displacement. Additionally, this lower resolution makes the bats appear longer.
Here are actual screenshots of the PAL (left) and NTSC (right) versions as created with my code (these match the actual original chips)...
Updated code and descriptions on my web page here...http://searle.hostei.com/grant/AVRPong/index.html
Any queries, as always, feel free to ask.