1. These are full [iso] images that were released w new appliances. On top of them one should install jumbo - jumbo 76 works on all (regular image and new images).
2. the numbers refer to full name (iso or jumbo) and the name include reference to jumbo. More importantly, once you install jumbo that is superset, its no longer important what image you used
3. Why two images? There were two separate steps of releasing new appliances and the jumbo that included all fixes didnt exist yet (obviously) so the base image of the new appliance contained fixes that were not at that time in jumbo 50 which was the last available and immediately after, we created jumbo 76 that include all fixed that are part of these images
4. so these are “base” image that you will find on the new appliances and if you add jumbo 76, you get to the same level of original ga plus jumbo 76. For all practical purposes you can see image 300 as replacing image 273 by adding more fixes, but we are transparent to the fact that both are out
Hope its as clear as it gets