I found this on Remington's website. I skipped right over when I looked before. Sorry about that.
NOTE: Model 870 Extra Barrels with 2-3/4" chambers will fit Super Magnum, Magnum, and 2-3/4" receivers.
NOTE: Model 870 Extra Barrels with 3" chambers will fit Super Magnum, Magnum, and 2-3/4" receivers. These barrels will handle both 2-3/4" and 3" shells when used on Magnum receivers. Use only 2-3/4" shells when these barrels are used on 2-3/4" receivers
Thanks anyway,
Paul