Parts Book PR953 (1971-1975) shows it on page 73.01.1. It looks like the rear half of a two-section exhaust pipe for a RHD R1123 (845cc) or R 2106 (F4 van). I think this was later replaced by a single-section pipe going all the way from the S-pipe at the front through to the rear outlet.
The corresponding front half was numbered 08 21 725 300.