My understanding is that the 2600 series was supposed to replace the 2500 series but they were building both the 2555 and 2655 with very few differences between them. It appears they've changed their minds and are now phasing out the 2655 and 2665 and replacing them with the 2660 and 2670 while also getting rid of the traditional shuttle transmission for those models and offering a PST transmission option instead. The PST would differentiate the offering from the 2555 and 2565 as well.
The engine is the same in the 55, 60, 65, and 70 models. They just detune the engine with the ECM to make the specified HP.
Also odd is that the Mahindra website still only indicates the 2655 and 2665 unless you go to the series pdf document. I believe they are trying to get the remaining 2655's and 2665's sold before they really start shipping the 2660's and 2670's.
As a side note, if you're looking at the 2600 series and want the HST transmission I don't think there's a convincing reason to wait for the 2660 instead of getting a 2655. The price will probably be slightly higher for that extra 5hp and if you're cutting your HP needs that close then you should be stepping up a size in tractor anyway. I have a 2655 and it's been a good strong tractor.