Speedfeed makes/made some of the synthetic stocks for Remington, as far as I know.
I prefer Speedfeed II sets for synthetic furniture on working guns, as those stocks have thicker sidewalls and are not so noisy or drum-like as the thin walled stocks tend to be.
If your serial number ends in X or N, a 12 gauge stock set should fit it perfectly. The older 20 ga. 870s were made on full size receivers.
hth,
lpl