I'm cetainly NOT a big Marlin 39 authority, but we have a couple of them around the house, one old model and one new model, so maybe I can comment on what I know...
For the past 40 or so years, the two main versions of the 39 that have been generally offered to the public have been the 39A and the 39M. The 39A is the Rifle with a curved lever and semi-pistol grip wrist on the stock and a rifle length 24" barrel. The 39M or "Mountie" version is similar, but has a 20" barrel and straight stock and lever.
The other 3 versions that I'm familiar with are the 39D, the 39TDS and the 1897.
The 1897 was a limited run rifle, made for a year or two where the Model 39 was renamed after its predecessor, the 1897 Marlin. This version had I believe a straight stock with checkering on both the wrist and forend and a half round/half octagonal barrel.
The 39TDS (Take Down Special or maybe "Safety"?) was a production run carbine with straight stock and lever, checkering on both the stock and forearm and 16-1/4" barrel and came with a short cordura case that would carry the Carbine broken-down into its two component groups.
I don't know exactly when the 39D was made, but I have one. It's an old model (pre-hammer block safety) 39 that essentially has the 39A stock with curved lever and the 39M 20" barrel.
The newer models have a hammer block safety in lieu of the half cock safety the old models had, and they also have a rebounding hammer.
This combination of changes makes the trigger letoff much heavier than the old models, and because of the rebounding hammer design, makes using lighter hammer springs less easy to use as it makes reliable ignition "spotty". The lighter Hammer Spring is one component used in obtaining a better trigger letoff IMHO.
