Does anybody know. I am looking to collect Pal exclusive NES games.

AFAIK just regional control for supply chain control. PAL is the format, A and B are the regions.

PAL-A games (Mattel or Nes-version) were sold in the UK and Italy, and PAL-B games in the rest of Europe (Scandinavia, Germany, France, Spain etc).
It's not possible to play PAL-A games in a PAL-B console and vice versa.

^ Right, that's what I thought, but the only reason why they are mutually exclusive is not hardware-based, it's purely because of the lockout.

So presumably, region-modding your native PAL console will give full glitch-free access to the other PAL region.

Thanks Guys. What about a Top Loader, then I don't have to worry about compatibility? 

That's my solution! Sounds like a plan.