It's a piggy-back cartridge...you have to plug in a normal NES game into the top connector to get it to work. The green version (which came with a cardboard standard-size box, if I recall correctly) is slightly rare, but the red version (which came with a Disney VHS-style plastic case) is scarce at best. Green versions generally go for a good bit more than the red versions, which themselves go for $10-$15 loose and $20-$25 complete.
HES is an unlicensed game company in Australia. They released Maxi 15, same as the US release made by AVE. Like in the US there are 2 versions: one with Double Strike and Pyramid (green label) and one with Death Race and Blackjack (red label). The green label Maxi 15 is a lot harder to find than the red label. Red labels Maxi 15 carts can also come in the cardboard box. I'd say the most common HES game is Maxi 15 (red label). You don't have to plug a normal NES cart into the top to make HES games work if played on a top loader system.
Yep that'd be it. There's actually 3 different types of HES carts. There's the "Piggy back" one in the picture linked, there's the 'Dongle' type where a dongle with a cable is plugged into the cart, and another cart is plugged into that. The third is just a normal cart. Toward the end of the NES era HES worked out how to get around the lockout chip, so they didn't need another cart connected.
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It's a piggy-back cartridge...you have to plug in a normal NES game into the top connector to get it to work. The green version (which came with a cardboard standard-size box, if I recall correctly) is slightly rare, but the red version (which came with a Disney VHS-style plastic case) is scarce at best. Green versions generally go for a good bit more than the red versions, which themselves go for $10-$15 loose and $20-$25 complete.