Nintendead

I have been picking up Atari 2600 games for a little while, but only recently got a Retron 77 to play them with. I am running Stella 6 Beta. While at a used game store the other day, I grabbed what I thought was a 2600 cartridge. The clerk told me it was actually an Intellivision cartridge. I know many Intellivision carts have an angled top with a label, and he said to look for a Mattel copyright to tell the difference. I had one cart like that at home, and assumed I bought an Intellivision game by mistake. After I hooked up my Retron 77, I looked at the cartridge (Dark Chambers) and it looked as if it was compatible. I put it in, and it worked. I know the 2600 was not compatible with Intellivision games, and I can't find anything about the Retron 77 or Stella 6 being able to run them. I also am having a lot of trouble finding images of the connecting side of Intellivision carts. I can only assume this was a game made by Mattel for the 2600, but can anyone tell me the best way to determine the difference going forward. Thanks.