The box art has been around for all three boxes since the system was still alive as shown in promo materials. Carts surfaced for two of them, and SNK USA was nearly as close to releasing Dynamite Slugger. I have a Dynamite Slugger cart with generic label I got from a SNK USA employee, and in the past I confirmed with an ESRB employee that Dynamite Slugger was rated E (without descriptors). Given the carts were produced in mass quantities and the box art had been produced, it seems plausible that there was some type of box produced. I think these are real boxes (retail, promo, or sample) from a test pressing.
It will be interesting to see if more pop up, from the same seller or elsewhere. Assuming these are real, this is the first time any of these have surfaced to my knowledge. From past experience, it seems when things like this pop up, they come in waves.
Yeah no way to prove it but I did buy Last Blade back in the day and it was new, it was in the box, but it wasn't like there was a stack of them lying around at the store either. I've seen the baseball one too, but never ever Faselei as I didn't think that one ever made it out of the UK complete. Really love to get back into this system but even the loose games on a few of the really good non-fighter ones I more enjoy have gotten a bit stupid (namely Biomotor Unitron.)
I don't think you are remembering correctly. I highly doubt you bought US versions of the game in official cardboard boxes, because they simply don't (or didn't) exist. You would be the only person in the world who ever bought these as such, which is unbelievable, to be honest.