Like I stated on the waterfowl forum, your receiver type is identified by the serial number suffix.
I just verified it from the remington website. If your serial # ends with a "V", it shoots 2 3/4" only. Ending with "M" shoots 2 3/4" and 3". A serial # with "A" shoots 2 3/4", 3", and 3 1/2". Go to
www.remington.com, go to Support, and click on Help Center to see for yourself.
Also, a 3" barrel will fit on a 2 3/4" only receiver and chamber a 3" shell. This practice is not advisable and I am not saying it is okay to use in this configuration. Just that it is possible to mix the two up.
Don't worry about the bismuth and tungsten shot yet. If you saved 4 years to buy your 870, you won't be able to afford these shells.