I am not an expert on the model 24, but the old model 25 had "regular" and "25 D" versions.
The standard model 25 had a simple 2-piece trigger/sear arrangement, while the 25D had the nice adjustable ball-sear trigger, "borrowed" from the larger model 27.
I don't know if they all have a letter code under the logo or not. It has a two screw trigger adjustment and a plastic trigger. The trigger is as crisp and light as either of my two R7s. The gun itself is way smoother and quieter than the R7s. What a little gem! I wonder if they are all this nice. I'm thinking of buying a 22 cal as well. It is a little low powered, barely making 600 fps with beeman laser pellets (177) and 525-540 with el cheapo crosman WCs.
A gun of this vintage with two screws in the trigger blade would be the classic ball-sear, I believe. Still can't say if that's what the "D" is for, but that would be my educated guess until somebody tells me differently!