Early 16 and 20 gauge 870s were all built on the 12 gauge frame. Later ones were made on the smaller receiver used today. Changeover occurred in the early 60s, IIRC.
I'm not sure when Remington dropped the 20 gauge on the 12 gauge frame but I have a 20 gauge Remington 870 Skeet that was made in 1978 that is on the 12 gauge frame.