What's a system you didn't know until it was retro? I have an Intellivision and an Atari 800XL, never used them as a kid, now they're mysterious and weird to me.
Never even heard of a Bally Astrocade until I joined up here. Still never seen one. I know if I do someday I'm gonna react like it's the flippin' Jade Falcon.
I had an Astrocade early on but never thought I'd find one as an adult. I hardly ever play it but I'd be sad if it wasn't there with the other soldiers.
The only thing I can think of is the Japanese equivalents of things, so I'm not sure if they count...Famicom, Super Famicom, Mega Drive and PC Engine...