The XX/XXI, later called the "Jr.", is different from the Spirit that replaced it. The Jr. is the same dimensions front to back as the full-size Genesis, but it is only about 2/3 as wide (if that). The middle burner was eliminated, but the front and back ones are still in the same position, just shorter. This grill DOES have TWO ROWS of flavorizer bars, just a few less in total due to the smaller size.
In spite of the double row of flavorizer bars, it has a reputation of not being the best cooker. Presumably this is because of the wide space between the two burners. When they came out with the 2-burner Spirit, they made the fire box more proportionately smaller. That reduces the space between the two burners, and it also makes the appearance less "cute" than the stubby Jr. This grill did a great job for us at the Meet this past Saturday, so I am not as sure that it deserves a bad reputation.
The one I got - thanks to Bruce's generous efforts to get it all the way from Minnesota - is a rare survivor that must have lived a charmed life away from the elements. Although the very first XX/XXI/Jr. grill had the single horizontal bottom frame bar like you described, this one has two, front and back, just like a regular Genesis. Considering that it is 33 years old, it is very solid. I consider myself very blessed to have such a cool piece of Weber history! It even came with the original owners manual and a working rotisserie with the correct size spit in perfect condition.