I was on a sabbatical pursuing other interests when 1.3 came out, so I never had it up and running but this is my understanding of it (mainly from Mr. Jelly I think}:
"EAW had always worked on a system of 30 aircraft slots. Any skin or 3dz files containing "p38h" were in the first slot, and any containing "V1V1" were in the 30th. This meant that the files were locked into slots. Not only that, but the flight model system was unhelpful. There were 30 individual flight model "flt" files, one for each slot, from "p38h.flt" to "V1V1.flt". Unfortunately other flight data files such as "planes.dat", "loadout.dat" and "pnames.str" covered all 30 slots. It was aggravating for modders.
However, during the development of 1.28D the Code Group found a way to break up those single files, such as "planes.dat", which contained data for all 30 aircraft slots. As a first stage, we were able to replace these with individual files for each plane. As a result we started working on an entirely new exe that could make use of this critical breakthrough. Principally through work done by Knegel, an exe was compiled which took the slot-free idea to its logical extent. The slots ceased to be named by or identified with the characteristics of particular aircraft (single, two engine etc). All thirty aircraft slots from 00 to 29 now had the full capacities available in any the previous particular slots. Any aircraft slot including the V1 slot could manage single and multi engine setups, props or jets.
This release was intended to be a public testing of the slot-free concept."
The great knock on from the slot free achievement was the introduction of new and additional 3dz "paste-on" files.
Have downloaded. Going to load them up in my 1914-18 Middle East setup. More later.