The significant differences between the model 27 and the model 28 are the 28's finish, the grips, sights,and the hammer and trigger.
The model 28 has what S&W called "satin blue". The 27 has "high luster blue".
The 27 has a checkered topstrap and grooved barrel rib (to reduce glare). The 28 is "bead blasted" (producing a matt surface finish to do the same thing).
The sights are standard blue on the 28, and all had the Baughman ramp front. 27s have the red ramp front, white outline rear, and often have a partridge front sight on the longer barrel models.
The trigger on the 28 is the standard narrow grooved service trigger, and the hammer is the service hammer, with a slightly larger pad on the spur than the usual service hammer found on most other models. The 27 has the wide target hammer and trigger.
As far as I can find out, all 28s left the factory with the small "magna" style grips. 27s have the larger target grips.
28s came in either 4" or 6". There is reported to have been a small number of special ordered 28s with 8 3/8" barrels (200 or maybe less guns) and a couple of special orders of uns in nickle finish, but aside from these small special orders all were satin blued.
28s never came with the presentation wood case. 27s usually did.
My favorite 28 has the target hammer and trigger. This was done sometimes back through the 70s when S&W offered blister packs to shops with the target parts. This upgrade was sometimes done to enhance the sale.
A 28 with a target hammer and trigger is about the best "economy" heavy revolver ever done, in my opinion.
