S'G' and IP marked receivers can be seen in both ranges. Barrel date is a good indicator of more or less IP parts. 4-43 vs 6-43

Earlier receivers had a rounded receiver ear (see data sheet) and later a square ear. I have not seen a square ear IP. I have not seen a 1.8 IP

I have a 3.21 S'G' at an auction I saw a IP that was only 9 digits away confirming that both blocks were produced by IP and reused/unused serial numbers were produced by S'G'