Through the late 1970s a Mirage would be, as MSwantak stated, 2040 Hi-Ten, I'm pretty positive of that as I bought the one step lower Nomade in 77 or 78 and it and the Mirage shared the same frame the difference being the Mirage had SunTour shifters and a 2 tone paint.
Though I obsessed on Motos during that time range I stopped during the 80s, eventually ending up on a Cannondale. However, from what I've seen since on eBay Moto expanded the Mirage label and some of the bikes could well be chro-moly or as T-Mar posted a Manganese alloy. Unless you dropped serious coin on ultra light wheels and maybe carbon bars I don't think you'd get a 2040 frame down to 23.5.
I swapped out a lot of parts on my Nomade which started at a claimed 29# and I doubt I got it much under 26# though it became a pretty nice ride.
