If you're confused by the description of two boxes and the picture of the single box, the picture is of the smaller box that was on top. The box on the bottom held the dynamite. From googling around a bit it wasn't exactly 1,000lbs of dynamite. It was 18 cases of Hercules Unigel dynamite, each case about 40lbs, so 720lbs.
Interestingly, the ransom note made the same factual error, calling the explosive TNT instead of dynamite. "If exploded this bomb contains enough TNT to severely damage Harrahs across the street. This should give you some idea of the amount of TNT contained within this box. It is full of TNT."
That's part of why they tried decapitating the device with a shaped charge. TNT isn't as easy to set off as dynamite is. According to a different account I read, if it had, as advertised, been TNT (and there weren't a few loose sticks in the top), that plan would have had a decent chance of working.
A tech-oriented rundown of the device: https://hackaday.com/2015/09/21/this-is-what-a-real-bomb-loo... (some factual errors, for example, there was no TNT in the device. Just dynamite. But a good overview of the anti-tamper pieces)