Not so, there are lots of differences.
An 870 Special field has a shorter mag tube and barrels will not interchange with anything else. The Supermags have larger ejection ports to work with longer shells. They made "Light Contour" barrels for a few years, standard barrels will not fit until you sand a bit of wood off the forend. Older guns had a ball detent system to hold the magazine cap in place, newer guns use a different system and an O ring or other modification needs to be made for barrels to interchange.