If you look at the back scan of the books you can see that it's advertising 'Pressman' (the toy company) board games. Within some of these games were 2nd prints of UXM #303, #307 (gold covers) & XM #11 (this was silver) and you could 'mail away' for #297 (gold), this is why, this is the rarest, I believe?
303 & 307 were mail-aways also. X-Men 11 was the only one in the box before they started the coupon submission.
Are you sure 303 & 307 were mail aways? I asked several Ebay sellers of the Pressman X-Men Alert game and they said theirs came with either the 303 or 307 already inside. As far as I know, only 297 had to be redeemed by mail.